fix for protocol-agnostic embedded URLs in content

Recently, YouTube changed the embed URLs provided, so that they are protocol agnostic;

<iframe width="560" height="315" src="//www.youtube.com/embed/bRdOBbluWgc" frameborder="0" allowfullscreen></iframe>

While <iframes> are, indeed, disallowed in *most* cases, the concept of a protocol-agnostic URL is not new, and continues to rise in prominence.

This fix should prevent URLs like the one shown above, from having JURI::base() concatenated to the beginning.