title: Use sinon stub for request constructor
body: <p>I have implement node js code to call HTTP API's using request module.
  For get, post, put, delete, I have directly called request() of require module,</p>
  
  <pre><code>var sendRequest = function(req, callback) {
      request(req, function(err, res) {
          if (err) {
              callback(err, null, res);
          } else {
              callback(null, res);
          }
      }
  }
  </code></pre>
  
  <p>I want to invoke sendRequest() but mock the response of request().
  Any pointers for this.</p>