ヘッダのカスタマイズや独自処理をしたいときの AFNetworking 2.0 の使い方 ref: http://qiita.com/gologo13/items/f36cb7208c15869888ca

file0.txt

AFHTTPRequestOperationManager *manager = [AFHTTPRequestOperationManager manager];
[manager GET:@"https://hogehoge.api.com/user"
  parameters:@{ @"user_id":@"hoge" }
     success:^(AFHTTPRequestOperation *operation, id responseObject) {
         NSLog(@"res:%@", responseObject);
     }
     failure:^(AFHTTPRequestOperation *operation, NSError *error) {
         NSLog(@"error:%@", error);
     }];

file1.txt

AFHTTPRequestOperationManager *manager = [AFHTTPRequestOperationManager manager];
[manager.requestSerializer setValue:@"SomeValue" forHTTPHeaderField:@"SomeHeaderField"]

file2.txt

- (void)setAuthorizationHeaderFieldWithUsername:(NSString *)username
                                       password:(NSString *)password;

...

[manager.requestSerializer setAuthorizationHeaderFieldWithUsername:@"username"
                                                            password:@"xxx"];

file3.m

AFHTTPRequestOperationManager *manager = [AFHTTPRequestOperationManager manager];

AFHTTPRequestSerializer *selializer = [AFHTTPRequestSerializer serializer];
self.requestSerializer = selializer;

NSString *URLString = @"https://api.twitter.com/1.1/friendships/create.json";

// リクエストに使う NSURLRequest を生成
// このメソッドを使うことで、AFNetworkingのパラメータのエンコードを良しなにやってくれる処理を再利用できる
NSMutableURLRequest *request =
   [selializer requestWithMethod:@"POST"
                       URLString:URLString
                      parameters:@{ @"screen_name":@"gologo13" }
                           error:nil];

// 独自の処理 リクエストヘッダに Twitter API を叩くためのアクセストークンを付与する
[[PFTwitterUtils twitter] signRequest:request];

// リクエスト開始
AFHTTPRequestOperation *operation = [self HTTPRequestOperationWithRequest:request success:success failure:failure];
[self.operationQueue addOperation:operation];