title: AFNetworking 2.0.0-RC2 invokes failure block not on main thread
body: <p>I got the AFNetworking version available on cocoapods (2.0.0-RC2) and in the AFURLSessionManager.m file the method do not invoke the failure block on the main thread in opposite of the success block. Is that on purpose or it is an error with the library?</p>
  
  <pre><code>- (NSURLSessionDataTask *)dataTaskWithRequest:(NSURLRequest *)request
                                    success:(void (^)(NSURLResponse *response, id responseObject))success
                                    failure:(void (^)(NSError *error))failure
  {
      NSURLSessionDataTask *task = [self.session dataTaskWithRequest:request completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
      if (error) {
          if (failure) {
              failure(error);
          }
      } else {
  ....
          id responseObject = [self.responseSerializer responseObjectForResponse:response data:data error:&amp;serializationError];
  
              dispatch_async(dispatch_get_main_queue(), ^(void) {
                  if (serializationError) {
                      if (failure) {
                          failure(serializationError);
                      }
                  } else {
                      if (success) {
                          success(response, responseObject);
                      }
                  }
  
  }
  </code></pre>