title: PromiseKit 2.0: chaining promises doesn't pass argument
body: <p>I have wrapped 2 of my API requests with Promise&lt;T&gt; return types:</p>
  
  <pre><code>func registerWithEmail(email: String, firstName: String, lastName: String, password: String, subscribe: Bool) -&gt; Promise&lt;Bool&gt;
  
  func requestAccessTokenWithEmail(username: String, password: String) -&gt; Promise&lt;String&gt;
  </code></pre>
  
  <p>Using them individually works fine:</p>
  
  <pre><code>firstly {
      client.registerWithEmail("foo@gmail.com", firstName: "john", lastName: "smith", password: "password", subscribe: false)
      }.then { success -&gt; Void in
          completion(success: success)
      }.catch { error -&gt; Void in
          println(error.localizedDescription)
  }
  </code></pre>
  
  <p>And:</p>
  
  <pre><code>firstly {
      client.requestAccessTokenWithEmail("foo@gmail.com", password: "password")
      }.then { accessToken -&gt; Void in
          println(accessToken)
          completion(success: true)
      }.catch { error -&gt; Void in
          println(error.localizedDescription)
  }
  </code></pre>
  
  <p>However, when I chain them, the token argument to the second <code>then</code> call is never populated:</p>
  
  <pre><code>firstly {
      client.registerWithEmail(username, firstName: "a", lastName: "b", password: password, subscribe: false).then { success -&gt; Void in
          return client.requestAccessTokenWithEmail(username, password: password)
      }.then { accessToken -&gt; Void in
          println(accessToken)
          completion(success: true)
      }
  }
  </code></pre>
  
  <p>If I break inside this closure I can't view the <code>accessToken</code> argument, only the outer <code>completion</code> closure. However, if I break inside the <code>requestAccessTokenWithEmail</code> function, the <code>accessToken</code> argument is populated when calling <code>fulfill</code>.</p>
  
  <p>I'm quite new to PromiseKit, so please let me know if I'm doing something stupid.</p>
  
  <p>I'm using PromiseKit 2.0, Swift 1.2, Xcode 6.4</p>