The user may wish to provide a dynamic queryset on a `RelatedField`
based on the `context`. The way to do that is to create a subclass of
`RelatedField` (or a child) and override the `get_queryset` method.
However, this is undocumented, and instantiating that field without a
`queryset` argument (because it's not needed) will raise an assertion
error.
Document `.get_queryset(self)` as an official part of the API of
`RelatedField`, and don't enforce the use of `queryset` when
`get_queryset` is overridden.
A PATCH to a serializer's non-related CharField was clearing an ancillary StringRelatedField(many=True) field.
The issue appears to be in the ManyRelatedField's get_value method, which was returning a [] instead of empty
when the request data was a MultiDict.
This fix mirrors code in fields.py, class Field, get_value, Ln. 272, which explicitly returns empty on a partial update.
Tests added to demonstrate the issue.
* Get rid of runtests.py
* Moved test code from rest_framework/tests and rest_framework/runtests to tests
* Invoke py.test from setup.py
* Invoke py.test from Travis
* Invoke py.test from tox
* Changed setUpClass to be just plain setUp in test_permissions.py
* Updated contribution guideline to show how to invoke py.test
