Fix html message attachment to `muc` message.

Pull request
[#26](https://github.com/metajack/strophejs-plugins/pull/26/files) for
fixing the XHTML support used `.h()` for attaching HTML message part.
Without it, it doesn't work.

Might be that you'd need to regenerate the .js file. I don't use CoffeeScript at all, therefore I just changed both sources.