title: Changing the text of a UITextField does not trigger the rx.text binder
body: <p>I have a <code>UITextField</code> called <strong>commentField</strong> and I create an <code>Observable&lt;Bool&gt;</code> like this:</p>
  
  <pre><code>let isCommentFieldValid = self.commentField.rx.text.orEmpty.map({ !$0.isEmpty })
  </code></pre>
  
  <p>This observable determines whether a button is enabled or not.
  The problem is that when I change the text property of <code>commentField</code> liked this:</p>
  
  <pre><code>self.commentField.text = ""
  </code></pre>
  
  <p>The <strong>isCommentFieldValid</strong> doesn't trigger again and, thus, the button's state doesn't change. Any edition using the UI works: if I remove all text from the field through the keyboard, the <strong>isCommentFieldValid</strong> updates, but via code it doesn't.</p>
  
  <p>Is there a reason this doesn't work?</p>