title: sinon - check if returned response object matches desired object
body: <p>I'm using sinon spy in order to spy on HTTP response status and send.
  The object I return in 'send' in json like so:</p>
  
  <pre><code>{
  message: 'bla bla',
  fields: { request_id ='000', user_id = '3434324' } 
  }
  </code></pre>
  
  <p>How can I checks if the returned response includes such an object, structure and values.</p>
  
  <p>Here's my code:</p>
  
  <pre><code>var responseStatusSpy = sandbox.spy(res, 'status');
  var responseSendSpy = sandbox.spy(res, 'send');
  var response_object = {message: "invalid request - no merchant id", fields: {'request_id':'0000', 'user_id' = '2323223'}};
  
  merchantController.getList(req, res);
  responseSendSpy.withArgs ?????
  </code></pre>
  
  <p>Thanks in advance.</p>