Returns super after setting the Content-Type header to
        ``application/json``

def http_method_not_allowed(self, *args, **kwargs):
        """
        Returns super after setting the Content-Type header to
        ``application/json``
        """
        resp = super(JsonResponseMixin, self).http_method_not_allowed(*args, **kwargs)
        resp['Content-Type'] = 'application/json'

        return resp