### Please describe the problem.

Need to annex some files from a http link which forwards to ftp.  addurl works out fine (besides not carrying about redirected filename), but drop fails to verify presence of the file


### What version of git-annex are you using? On what operating system?

6.20160706+gitgc4229be-1~ndall+1

### Please provide any additional information below.

[[!format sh """
smaug:/tmp
$> mkdir /tmp/123; cd /tmp/123; git init; git annex init;
Initialized empty Git repository in /tmp/123/.git/
init  ok
(recording state in git...)
1 10080.....................................:Tue 12 Jul 2016 02:18:46 PM EDT:.
(git)smaug:/tmp/123[master]
$> git annex addurl --debug --pathdepth=-1 http://www.nitrc.org/frs/downloadlink.php/1637
[2016-07-12 14:18:54.8195] read: git ["--git-dir=.git","--work-tree=.","--literal-pathspecs","show-ref","git-annex"]
[2016-07-12 14:18:54.83021] process done ExitSuccess
[2016-07-12 14:18:54.830357] read: git ["--git-dir=.git","--work-tree=.","--literal-pathspecs","show-ref","--hash","refs/heads/git-annex"]
[2016-07-12 14:18:54.841375] process done ExitSuccess
[2016-07-12 14:18:54.841694] read: git ["--git-dir=.git","--work-tree=.","--literal-pathspecs","log","refs/heads/git-annex..787586641a9027e772b91989eb649cb9303cd975","--pretty=%H","-n1"]
[2016-07-12 14:18:54.851263] process done ExitSuccess
[2016-07-12 14:18:54.852043] chat: git ["--git-dir=.git","--work-tree=.","--literal-pathspecs","cat-file","--batch"]
[2016-07-12 14:18:54.862122] read: quvi ["--version"]
[2016-07-12 14:18:54.869939] process done ExitSuccess
[2016-07-12 14:18:54.870062] call: quvi ["--verbosity","mute","--support","http://www.nitrc.org/frs/downloadlink.php/1637"]
[2016-07-12 14:18:54.891339] process done ExitFailure 65
addurl 1637 (downloading http://www.nitrc.org/frs/downloadlink.php/1637 ...) 
[2016-07-12 14:18:55.830611] call: wget ["-q","--show-progress","--clobber","-c","-O","/tmp/123/.git/annex/tmp/URL--http&c%%www.nitrc.org%frs%downloadlink.php%1637","http://www.nitrc.org/frs/downloadlink.php/1637","--user-agent","git-annex/6.20160706+gitgc4229be-1~ndall+1"]
1637                                         100%[===========================================================================================>] 268.32M  12.9MB/s    in 27s     
[2016-07-12 14:19:31.32811] process done ExitSuccess
[2016-07-12 14:19:31.328909] chat: git ["--git-dir=.git","--work-tree=.","--literal-pathspecs","check-attr","-z","--stdin","annex.backend","annex.numcopies","annex.largefiles","--"]
[2016-07-12 14:19:31.329444] read: git ["--version"]
[2016-07-12 14:19:31.338195] process done ExitSuccess
[2016-07-12 14:19:31.338947] read: sha256sum [".git/annex/tmp/URL--http&c%%www.nitrc.org%frs%downloadlink.php%1637"]
[2016-07-12 14:19:32.860187] process done ExitSuccess
ok
(recording state in git...)
[2016-07-12 14:19:32.872749] feed: xargs ["-0","git","--git-dir=.git","--work-tree=.","--literal-pathspecs","add","--"]
[2016-07-12 14:19:32.890096] process done ExitSuccess
[2016-07-12 14:19:32.890573] chat: git ["--git-dir=.git","--work-tree=.","--literal-pathspecs","hash-object","-w","--stdin-paths","--no-filters"]
[2016-07-12 14:19:32.891376] feed: git ["--git-dir=.git","--work-tree=.","--literal-pathspecs","update-index","-z","--index-info"]
[2016-07-12 14:19:32.901206] process done ExitSuccess
[2016-07-12 14:19:32.901288] read: git ["--git-dir=.git","--work-tree=.","--literal-pathspecs","show-ref","--hash","refs/heads/git-annex"]
[2016-07-12 14:19:32.909396] process done ExitSuccess
[2016-07-12 14:19:32.909691] read: git ["--git-dir=.git","--work-tree=.","--literal-pathspecs","write-tree"]
[2016-07-12 14:19:32.91827] process done ExitSuccess
[2016-07-12 14:19:32.918324] chat: git ["--git-dir=.git","--work-tree=.","--literal-pathspecs","commit-tree","6c1ce1871151c12f6930733e7693eac2fd862677","--no-gpg-sign","-p","refs/heads/git-annex"]
[2016-07-12 14:19:32.927217] process done ExitSuccess
[2016-07-12 14:19:32.927281] call: git ["--git-dir=.git","--work-tree=.","--literal-pathspecs","update-ref","refs/heads/git-annex","2453a1c23977e4f17ac5fd9668cdc7bdb93e34e8"]
[2016-07-12 14:19:32.936074] process done ExitSuccess
cached/staged changes:                                                                                                                                                           
 1637 | 1 +
1 10081.....................................:Tue 12 Jul 2016 02:19:32 PM EDT:.
(git)smaug:/tmp/123[master]
$> git commit -m 'added'                                                          
[master (root-commit) b212851] added
 1 file changed, 1 insertion(+)
 create mode 120000 1637
1 10082.....................................:Tue 12 Jul 2016 02:19:38 PM EDT:.
(git)smaug:/tmp/123[master]git
$> git annex drop 1637                                                            
drop 1637 (checking http://www.nitrc.org/frs/downloadlink.php/1637...) (unsafe) 
  Could only verify the existence of 0 out of 1 necessary copies

  Rather than dropping this file, try using: git annex move

  (Use --force to override this check, or adjust numcopies.)
failed
git-annex: drop: 1 failed
1 10083 ->1.....................................:Tue 12 Jul 2016 02:19:48 PM EDT:.
(git)smaug:/tmp/123[master]git
$> wget -S http://www.nitrc.org/frs/downloadlink.php/1637
--2016-07-12 14:19:54--  http://www.nitrc.org/frs/downloadlink.php/1637
Resolving www.nitrc.org (www.nitrc.org)... 132.239.16.23
Connecting to www.nitrc.org (www.nitrc.org)|132.239.16.23|:80... connected.
HTTP request sent, awaiting response... 
  HTTP/1.1 302 Found
  Date: Tue, 12 Jul 2016 18:19:54 GMT
  Server: Apache/2.2.15 (CentOS)
  X-Powered-By: PHP/5.3.3
  Set-Cookie: PHPSESSID=vhcpo1fmi205cfv0h4jgbnn9a0; path=/
  Expires: Thu, 19 Nov 1981 08:52:00 GMT
  Cache-Control: no-store, no-cache, must-revalidate, post-check=0, pre-check=0
  Pragma: no-cache
  Location: ftp://www.nitrc.org/fcon_1000/htdocs/Ontario.tar
  Content-Length: 0
  Keep-Alive: timeout=2, max=100
  Connection: Keep-Alive
  Content-Type: text/html; charset=UTF-8
Location: ftp://www.nitrc.org/fcon_1000/htdocs/Ontario.tar [following]
--2016-07-12 14:19:55--  ftp://www.nitrc.org/fcon_1000/htdocs/Ontario.tar
           => ‘Ontario.tar’
Connecting to www.nitrc.org (www.nitrc.org)|132.239.16.23|:21... connected.
Logging in as anonymous ... 
220 (vsFTPd 2.2.2)
--> USER anonymous

331 Please specify the password.
--> PASS Turtle Power!

230 Login successful.
--> SYST

215 UNIX Type: L8
--> PWD

257 "/"
--> TYPE I

200 Switching to Binary mode.
--> CWD /fcon_1000/htdocs

250 Directory successfully changed.
--> SIZE Ontario.tar

213 281354240
--> PASV

227 Entering Passive Mode (132,239,16,23,106,251).
--> RETR Ontario.tar

"""]]

[[!meta author=yoh]]

> [[fixed|done]] --[[Joey]]