Fix "not well-formed" error in Firefox.
Fix "not well-formed" error in Firefox.

Firefox requires the mime type to be set on the XMLHttpRequest used to
load javascript modules (see http://stackoverflow.com/a/677943/1005455),
otherwise it spits a "not well-formed" error that pollutes the console.
I'm patching Yabble directly.

jrburga/jsvgdl,jrburga/jsvgdl,GameJs/gamejs,GameJs/gamejs,jrburga/jsvgdl,jrburga/jsvgdl,GameJs/gamejs