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

`_resolve_model` should work with unicode strings

fields: allow help_text on SerializerMethodField

...by passing through any extra *args and **kwargs
to the parent constructor.

Previously one couldn't assign help_text to a
SerializerMethodField during construction.

Allow customising ChoiceField blank display value

fix pep8