Replace uses of the "mimetype" arg of HttpResponse with "content_type".

[korfuri]

mimetype has been deprecated for a long time and has been removed from
django in the development version. This commit uses content_type for
the default value but allows specifying either, regardless of the
version of Django. Specifying "mimetype" will work for users of
django-oauth2-provider if their version of django supports it (the
deprecated argument has precedence), and will break if it doesn't.

[jarcoal]

+1

[deronnax]

+1
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__

[gio82]

+1

[pkozela]

+1

[korfuri]

@eculver can you comment on this? Thanks!

[psychok7]

+1 , i need this to work with Django 1.7

[RogerThomas]

+1 Any chance this can be merged soon?