work around Network.Browser bug that converts a HEAD to a GET when following a redirect

The code explicitly switches from HEAD to GET for most redirects.
Possibly because someone misread a spec (which does require switching from
POST to GET for 303 redirects). Or possibly because the spec really is that
bad. Upstream bug: https://github.com/haskell/HTTP/issues/24

Since we absolutely don't want to download entire (large) files from
the web when checking that they exist with HEAD, I wrote my own redirect
follower, based closely on the one used by Network.Browser, but without
this misfeature.

Note that Network.Browser checks that the redirect url is a http url
and fails if not. I don't, because I want to not need to change this
code when it gets https support (related: I'm surprised to see it
doesn't support https yet..). The check does not seem security significant;
it doesn't support file:// urls for example. If a http url is redirected
to https, the Network.Browser will actually make a http connection again.
This could loop, but only up to 5 times.

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