Fixes an issue with the request serializer silently handling an error

When a query string serialization block is set on
`AFHTTPRequestSerializer`, if it was to fail with an error it would be
silently handled and the serialization would still return an NSURLRequest.
Merge branch 'AFURLRequestSerialization-error' of github.com:kylef/AFNetworking into kylef-AFURLRequestSerialization-error

Conflicts:
	Tests/Tests/AFHTTPRequestSerializationTests.m