title: iframe not working in tinymce when valid_elements defined
body: <p>As soon as I define any valid_elements and/or extended_valid_elements I cannot insert iframe (iframe defined among valid_elements, extended_valid_elements)</p>
  
  <p>All online samples with iframe support and valid_elements are with tinymce v3
  and in v3 it works <a href="http://fiddle.tinymce.com/cTeaab/2" rel="nofollow">http://fiddle.tinymce.com/cTeaab/2</a></p>
  
  <p>But in v4 it does not <a href="http://jsfiddle.net/armandsp/ug247bsm/" rel="nofollow">http://jsfiddle.net/armandsp/ug247bsm/</a></p>
  
  <pre><code>valid_elements : "a[href|target=_blank],strong,u,p,iframe[src|frameborder|style|scrolling|class|width|height|name|align]",
  extended_valid_elements : "iframe[src|frameborder|style|scrolling|class|width|height|name|align]"
  </code></pre>