title: How to return a modified result in a promise as a promise
body: <p>I'm using promiseKit 6 with the extensions for Alamofire. In the function bellow I'm looking to return <code>Promise&lt;JSON&gt;</code> (I use swiftyJson) but the response from the alamofire call is a Promise containing a tuple: <code>Promise&lt;(json: Any, response: PMKAlamofireDataResponse)&gt;</code></p>
  
  <p>when I get that from the first then, how can I continue to return just the json part? thanks :)</p>
  
  <pre><code>return firstly {
    self.requestClient.request(url, method: .get, parameters: nil, encoding: JSONEncoding.default, headers: nil).responseJSON()
  }.then { arg in
    let (json, rsp) = arg
    return json
  }
  </code></pre>
  
  <p>I also get this error: <code>Cannot convert return expression of type 'Promise&lt;_.T&gt;' to return type 'Promise&lt;JSON&gt;'</code>
  on the line: <code>}.then { arg in ...</code></p>