Fix wrong content type sent with ajax calls.

Most of the calls to the ReST api were broken in django 1.5 as a result
of sending application/json in the request on POSTs.

Reference:
https://docs.djangoproject.com/en/dev/releases/1.5/#non-form-data-in-http-requests

Added a unit test to check returns of top_alert_producers although that
was not triggered during this specific issue.

Fixes #422