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
Replace ChoiceField type_label
Merge pull request #1619 from khamaileon/master

Replace ChoiceField type_label