prevent infinite-loops when loading mercury editor

When I followed "Rails Integration Techniques", there is a problem with loading Mercury editor.

When the page is ended with #, Mercury editor will be loaded even in the iframe, which creates an infinite-loops of loading the editor.

This happened because param mercury_frame was not set, due to being after the hash "#" symbol.

The pull request just adds a replace call to remove the trailing hash in iframeSrc url.