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.

Author: mweibel

muc/strophe.muc.coffee

@@ -164,7 +164,7 @@ Strophe.addConnectionPlugin 'muc',
     if html_message?
       msg.c("html", xmlns: Strophe.NS.XHTML_IM)
       .c("body", xmlns: Strophe.NS.XHTML)
-      .t(html_message)
+      .h(html_message)
       if msg.node.childNodes.length is 0
         # html creation or import failed somewhere; fallback to plaintext
         parent = msg.node.parentNode