views: Replace 'mimetype' with 'content_type'

views: Replace 'mimetype' with 'content_type'

Passing 'mimetype' to 'HttpResponse' is deprecated in 1.6 and
removed in Django 1.7. Among other things, this causes some unit
tests to fail when using Django 1.7. Its replacement - 'content_type'
- is available in Django 1.5+. This can be seen here:

    https://docs.djangoproject.com/en/1.5/ref/request-response/#django.http.HttpResponse.__init__

This is therefore a like-for-like replacement.

Signed-off-by: Stephen Finucane <stephenfinucane@hotmail.com>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>