[#632] Add NonNullCharField
[#632] Add NonNullCharField

The NonNullCharField patches the behaviour of the standard CharField, it
returns u'' if the value is None.

See discussion at
https://github.com/tomchristie/django-rest-framework/pull/1665/. Also
note that it seems the whole Serializers part of DRF is up for a
refactoring so this is to be considered a temporary fix (hopefully).

akvo/akvo-rsr,akvo/akvo-rsr,akvo/akvo-rsr,akvo/akvo-rsr