title: NSURLRequest setValue: forHTTPHeaderField:@"Authorization" not working
body: <p>I try to make an authenticate call to the twitter API with the <strong>Application only Authentication API</strong></p>
  
  <pre><code>NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:[NSURL URLWithString:url] 
                                                         cachePolicy:NSURLRequestReloadIgnoringLocalCacheData 
                                                     timeoutInterval:20.0];
  [request setHTTPMethod:@"POST"];
  [request setValue:@"application/x-www-form-urlencoded;charset=UTF-8" forHTTPHeaderField:@"Content-Type"];
  [request setValue:@"Some App Name" forHTTPHeaderField:@"User-Agent"];
  [request setValue:authValue forHTTPHeaderField:@"Authorization"];
  [request setHTTPBody:[@"grant_type=client_credentials" dataUsingEncoding:NSUTF8StringEncoding]];
  </code></pre>
  
  <p>Interestingly the <strong>authValue</strong> is not getting set for HTTPHeaderField @"Authorization". Therefor my calls fail of course. All other header fields are set correctly.</p>
  
  <p>I check it by</p>
  
  <pre><code>NSLog(@"Authorization, %@, %@", [request valueForHTTPHeaderField:@"Authorization"], authValue);
  </code></pre>
  
  <p>which returns <strong>@"Authorization, (null), theCorrectStuff</strong></p>
  
  <p>Why is this happening? I should be able to set it right away, or am I missing something?</p>
  
  <p>Thanks for your time and consideration,
  Fabian</p>