Fixed compiler warnings for upcoming version of Xcode
Session manager: Fixes a bug with updating download progress

Instead of updating the completed progress, it was instead incremementing the
total progress.

Related #1780
Only remove observer if we're observing

We’re only observing this object if we have a delegate for it

Closes #1763
Merge pull request #1779 from synchromation/master

Fixed compiler warnings for upcoming version of Xcode
Merge pull request #1769 from kylef/remove-observer

Only remove observer if we're observing
Merge branch 'bug/progress-incorrect' of github.com:kylef/AFNetworking into kylef-bug/progress-incorrect

Conflicts:
	AFNetworking/AFURLSessionManager.m
[Issue #1782] Fixing AFHTTPRequestOperationManager to default shouldUseCredentialStorage to YES, as per documentation
Merge remote-tracking branch 'AFNetworking/AFNetworking/master'

Conflicts:
	AFNetworking/AFURLSessionManager.m