Use content_type instead of deprecated mimetype

[zarelit]

Fixes compatibility with Django 1.8

[Brightcells]

Just as deronnax commented at caffeinehit/django-oauth2-provider#84

• The deprecation in 1.5 : https://docs.djangoproject.com/en/1.5/ref/request-response/#django.http.HttpResponse.__init__
• Removal effective in 1.7 : https://docs.djangoproject.com/en/1.7/ref/request-response/#django.http.HttpResponse.__init__