[Issue #737] Updating default Accept-Language header value in AFHTTPClient
Fixes warning: multiple methods named 'postNotificationName:object:' found [-Wstrict-selector-match]
Fixes warning: multiple methods named 'objectForKey:' found [-Wstrict-selector-match]
Fixes warning: weak receiver may be unpredictably set to nil [-Wreceiver-is-weak]

Signed-off-by: Mattt Thompson <m@mattt.me>
Compare NSURLRequest rather than NSURL

If you inadvertently load a nil URL in UIImageView+AFNetworking, the failure block of the image request operation is bypassed because [nil isEqual:nil] == NO. By comparing the URL request instead of the URL, the `isEqual:` test passes and the error is properly forwarded to the failure block parameter.
Limiting default Accept-Language header to 6 languages
Merge branch 'experimental-issue-737'

Conflicts:
	AFNetworking/AFHTTPClient.m
Adding lock around AFHTTPRequestOperation -responseString
Adding lock around AFURLConnectionOperation -responseStringEncoding

Adding check for self.response in AFURLConnectionOperation -responseStringEncoding
Merge pull request #772 from 0xced/fix-uiimageview-nil-url

Compare NSURLRequest rather than NSURL
Fixing warnings caused by implicit property attributes
Ignoring -Wcovered-switch-default in AFHTTPBodyPart -hasBytesAvailable
Merge branch 'master' of github.com:AFNetworking/AFNetworking
Using DISPATCH_QUEUE_CONCURRENT for all calls to dispatch_queue_create for media type processing queues
Use dispatch_once to initialize content-type-specific processing queues

Signed-off-by: Mattt Thompson <m@mattt.me>