Remove poster attribute instead of setting to null

Setting the poster property to null is equivalent to settings the attribute value to "". Unlike most other browsers Firefox correctly interpretes "" as a valid relative URL and starts another request to the current page (or it's base URL). Firefox has adopted other browsers' behaviour and ignores empty <img> src attributes now (https://bugzilla.mozilla.org/show_bug.cgi?id=444931), but not yet so for empty poster attributes. This means that poster="" potentially breaks pages using, for example, CSRF protection and that the attribute has to be removed from the tag instead, which should have the same effect to the Safari/Chrome bugs mentioned.

Short example: http://jsfiddle.net/eCZFy/