title: Error: 'String' is not convertible to 'String!'
body: <pre><code>mapView.rac_valuesForKeyPath("userTrackingMode", observer: self).subscribeNextAs { 
  // block handling
  </code></pre>
  
  <p>I get an error <code>'String' is not convertible to 'String!'</code>. Any suggestions what this may mean?</p>
  
  <p>I used to think, that <code>String!</code> is same as <code>String</code>, so it is unwrapped <code>String?</code>...</p>
  
  <ul>
  <li>Xcode 7.3.1</li>
  <li>Swift 2.2</li>
  <li>ReactiveCocoa 4.1.0</li>
  </ul>