Merge pull request #1603 from khamaileon/master

Add the allow_add_remove parameter to the get_serializer method
Note on configuring TokenAuthentication
Always use specified content type in APIRequestFactory

If `content_type` is specified in the `APIRequestFactory`, always
include it in the request, even if data is empty.
Merge pull request #1608 from dbrgn/test_client_contenttype

Always use specified content type in APIRequestFactory
Fix typo in docs
Merge pull request #1612 from rhcarvalho/patch-3

Fix typo in docs