title: AFHTTPSessionManager header
body: <p>I am trying to set a default header for "Content-Type" by setting HTTPAdditionalHeaders. When I look at the request header, AFNetworking (v 2.0.3) changes it back. I also tried to set header by setValue:forHTTPHeaderField: on the requestSerializer, but no success. What I am missing? </p>
  
  <p><strong>UPDATED</strong></p>
  
  <pre><code>    NSURL *URL = [NSURL URLWithString:@"http://example.com/api"];
  
      NSURLSessionConfiguration *configuration = [NSURLSessionConfiguration defaultSessionConfiguration];
      configuration.HTTPAdditionalHeaders = @{@"Content-Type": @"multipart/form-data"};
  
      AFHTTPSessionManager *manager = [[AFHTTPSessionManager alloc] initWithBaseURL:URL sessionConfiguration:configuration];
      manager.responseSerializer = [AFJSONResponseSerializer serializer];
  
      NSMutableDictionary *params = [[NSMutableDictionary alloc]init];
      [params setValue:@"some value" forKey:@"someKey"];
  
      [manager POST:@"search" parameters:params success:^(NSURLSessionDataTask *task, id responseObject) {
          NSLog(@"success");
      } failure:^(NSURLSessionDataTask *task, NSError *error) {
          NSLog(@"error");
      }];
  </code></pre>