Enumerator using generic. Cleaner for .. in .. loop in Swift (#433)

* Enumerator using generic. Cleaner for .. in .. loop in Swift

So 
for childSnapshot in snapshot.children {
you don't need to cast each child. Each cild is already known as DataSnapshot

* Update FIRDataSnapshot.h

* Update FIRMutableData.h

* Update FIRMutableData.m

2 files changed, 2 insertions, 2 deletions

diff --git a/Firebase/Database/Api/FIRDataSnapshot.m b/Firebase/Database/Api/FIRDataSnapshot.m
index 9559c38..b774493 100644
--- a/Firebase/Database/Api/FIRDataSnapshot.m
+++ b/Firebase/Database/Api/FIRDataSnapshot.m
@@ -87,7 +87,7 @@
     return [self.node.node numChildren];
 }
 
-- (NSEnumerator *) children {
+- (NSEnumerator<FIRDataSnapshot *> *) children {
     return [[FTransformedEnumerator alloc] initWithEnumerator:self.node.childEnumerator andTransform:^id(FNamedNode *node) {
         FIRDatabaseReference *childRef = [self.ref child:node.name];
         return [[FIRDataSnapshot alloc] initWithRef:childRef indexedNode:[FIndexedNode indexedNodeWithNode:node.node]];
diff --git a/Firebase/Database/Api/FIRMutableData.m b/Firebase/Database/Api/FIRMutableData.m
index 7e10dcd..77a022a 100644
--- a/Firebase/Database/Api/FIRMutableData.m
+++ b/Firebase/Database/Api/FIRMutableData.m
@@ -110,7 +110,7 @@
     return [self.data getNode:self.prefixPath];
 }
 
-- (NSEnumerator *) children {
+- (NSEnumerator<FIRMutableData *> *) children {
     FIndexedNode *indexedNode = [FIndexedNode indexedNodeWithNode:self.nodeValue];
     return [[FTransformedEnumerator alloc] initWithEnumerator:[indexedNode childEnumerator] andTransform:^id(FNamedNode *node) {
         FPath* childPath = [self.prefixPath childFromString:node.name];