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.
Merge pull request #14 from mweibel/muc/fix-html-message

Fix html message attachment to `muc` message.