title: textViewShouldBeginEditing not called when not editable become to editable
body: <p>I encountered a problem on the UITextFieldDelegate, anyone can help me will be great appreciate.
  There is a UITextView, and I implemented its delegate. Normally all delegate methods called very good. But in this case:</p>
  
  <ol>
  <li>set the UITextView not editable.</li>
  <li>long-press the UITextView until the popup appear (copy, cut, ...).</li>
  <li>dismiss the popup, and set the UITextView editable</li>
  <li>tap the UITextView to make the UITextView enter editing mode.</li>
  <li>you will find the delegate method textViewShouldBeginEditing not called.</li>
  </ol>
  
  <p>I need the textViewShouldBeginEditing be called to handle some UI changes. Anyone knows how to solve it? Thanks very much!</p>
  
  <p><strong>Test project</strong>: I created a simple test project which will NSLog the calls of the methods, you can test my case quickly, thanks! The source code is here: <a href="http://goo.gl/tGQS5" rel="noreferrer">http://goo.gl/tGQS5</a></p>