wptserve.py should send JSON failure message with correct mime type.
wptserve.py should send JSON failure message with correct mime type.

According to the JSON RFC [1] the official mime type is "application/json". If
we do not send this, Gecko will not fire the onload event for a frame trying to
load such a URL but will instead load an error page. Gecko bug discussion is
at [2].

Several service worker tests rely on loading 'fake' iframes that 404 but
contribute to the progress of the test by watching for the onload event.

[1]: http://www.ietf.org/rfc/rfc4627.txt
[2]: https://bugzilla.mozilla.org/show_bug.cgi?id=1201632
youennf/wptserve