Renamed the 'mimetype' arg to 'content_type'

'content_type' was introduced in Django 1.0 to replace 'mimetype', they kept the compatibility until 1.5 but it's now gone in 1.6.
See https://github.com/django/django/commit/8eadbc5a03d06f5bfedfa3fad35ad0801d2ab6ff and https://docs.djangoproject.com/en/1.6/ref/request-response/#django.http.HttpResponse.__init__
Merge pull request #389 from tdruez/patch-1

Renamed the 'mimetype' arg to 'content_type'