Replace deprecated mimetype with content_type

Passing `mimetype` to an HttpResponse object is deprecated
since Django 1.5 and will throw an error in Django 1.7.
`content_type` is supported since Django 1.0.

Author: foonicorn

adminsortable/admin.py

@@ -246,7 +246,7 @@ def do_sorting_view(self, request, model_type_id=None):
                 pass
 
         return HttpResponse(json.dumps(response, ensure_ascii=False),
-            mimetype='application/json')
+            content_type='application/json')
 
 
 class SortableInlineBase(SortableAdminBase, InlineModelAdmin):