Merge branch 'master' of https://github.com/tomchristie/django-rest-framework into feature/filepathfield

Helpful error on erronous 'serializer.save(commit=False)'

Upgrade guardian support to 1.3.

Removed import of unused django.forms.widgets

Added allow_empty flag

Test for #3153

Runtests should not suppress stdout. In the context of a web app only serves to hide erronous print statements.

Form fields should use textual only value representations. Closes #3139. Closes #2416. Closes #2558.

Raise helpful error when erronously including nested data in multipart post requests with test client. Closes #2919.

Handle .choices for related fields when queryset argument is actually a mamanger instance. Closes #3145.

Ensure that realtionship fields '.choices' returns an empty dict when accessed with a read-only field. Closes #2910.

Add `get_page_size` hook to `CursorPagination`