Fixes iframe load handling when src empty/unspecified

Fixes [issue #749](https://github.com/tmpvar/jsdom/issues/749) by setting url to 'about:blank' and loading up an empty document, as per the [spec](http://dev.w3.org/html5/spec/Overview.html#the-iframe-element).  Also addresses an issue with the initial insertion handler, which was not being properly removed when the src attribute was encountered during processing.