Fixed encoding parameter in QueryDict

authentication: allow all transport modes of access token in OAuth2Authentication

Update serializers.py

Prevent iterating over a string that is supposed to be an iterable <==> Prevent read_only_fields = ('some_string)

Prevent iterating over a string that is supposed to be an iterable <==> Prevent read_only_fields = ('some_string)

RFC6750 describe three transport modes for access tokens when accessing a
protected resource:
- Auhthorization header with the Bearer authentication type
- form-encoded body parameter
- URI query parameter

This patch add support for last two transport modes.

Serializer fields section for 3rd-party packages

* Add new section to serializer fields page where we can list and link 3rd-party packages that provide more field types
* Add an entry for drf-compound-fields

Reintroduced url arguments in the urls for the tests.

Add SEARCH_PARAM and ORDERING_PARAM to settings

Unique constraint are validated even if the field is optional.

Fixes #1434

don't implicitly import provider.oauth2

Fix typo

Update documentation of render methods (obj to data)