title: Share extension - beginRequestWithExtensionContext not called
body: <p>I've have a share extension for an iOS app that has a custom UI. When running the extension the storyboard is loaded and the UI presented OK, but <code>beginRequestWithExtensionContext</code> isn't called.</p>
  
  <p><strong>The (simplified) view controller code:</strong></p>
  
  <pre><code>class ShareViewController: UIViewController, NSExtensionRequestHandling {
  
      // This is called…
      override func viewDidLoad() {
          super.viewDidLoad() 
      }
  
      // …but this isn't!
      override func beginRequestWithExtensionContext(context: NSExtensionContext) {            
          println("beginRequestWithExtensionContext")
      }
  }
  </code></pre>
  
  <p><strong>NSExtensionAttributes</strong>:</p>
  
  <pre><code>&lt;dict&gt;
      &lt;key&gt;NSExtensionAttributes&lt;/key&gt;
      &lt;dict&gt;
          &lt;key&gt;NSExtensionActivationRule&lt;/key&gt;
          &lt;dict&gt;
              &lt;key&gt;NSExtensionActivationSupportsImageWithMaxCount&lt;/key&gt;
              &lt;integer&gt;1&lt;/integer&gt;
          &lt;/dict&gt;
      &lt;/dict&gt;
      &lt;key&gt;NSExtensionPointIdentifier&lt;/key&gt;
      &lt;string&gt;com.apple.share-services&lt;/string&gt;
      &lt;key&gt;NSExtensionMainStoryboard&lt;/key&gt;
      &lt;string&gt;Share&lt;/string&gt;
  &lt;/dict&gt;
  &lt;/plist&gt;
  </code></pre>
  
  <p>Any thoughts as to why <code>beginRequestWithExtensionContext</code>  might not be getting called?</p>