Update fields.md

obj.__class__ will return the actual Class object, we want to serialise a string (accessed with obj.__class__.__name__)

Modelserialization charfield with null

Fixes #1651. Add link to drf-extensions nested routers to docs

tiny typo in the docstring

Version 2.3.14

Fix #1614 - Corrected reference to serializers.CharField

Replace ChoiceField type_label

Fix typo in docs

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.

Add the allow_add_remove parameter to the get_serializer method