title: json response is interpreted as text/plain
body: <p>I have the following code:</p>
  
  <pre><code>NSURL *URL = [NSURL URLWithString:[@"some-address"]];
  NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:URL];
  
  RKObjectRequestOperation *requestOperation = [[RKObjectRequestOperation alloc] initWithRequest:request responseDescriptors:[self.objectManager responseDescriptors]];
  
  [requestOperation start];
  [requestOperation waitUntilFinished];
  </code></pre>
  
  <p>I get the following error.</p>
  
  <pre><code>Object request failed: Underlying HTTP request operation failed with error: Error Domain=AFNetworkingErrorDomain Code=-1016 "Expected content type {(
      "application/x-www-form-urlencoded",
      "application/json"
  )}, got text/plain" UserInfo=0x1f5e3c40 {NSLocalizedRecoverySuggestion={"total_rows":16,"offset":1,"rows":[
  
  {"id":"1","key":1,"value":{"_id":"1","_rev":"1-e75042683867a7030fc4d3aa3b72ef35",
  "user":{
  "userId":"1",
  "name":"A",
  .......
  ]}}, .....
  </code></pre>
  
  <p>Why I get this error, when the response is in Json format?</p>