Fix deprecation warning for mimetype keyword argument

This applies only to develop (Django 1.6)

--no-rebase

<pre>
/omero/openmicroscopy/dist/lib/python/django/http/response.py:330: DeprecationWarning: Using mimetype keyword argument is deprecated, use content_type instead
  super(HttpResponse, self).__init__(*args, **kwargs)
</pre>