A model's field is required if is null or blank
Adding `BLANK_CHOICE_DASH` as a choice if the model's field isn't required
Merge branch 'master' into 725-blank-choice-dash

Conflicts:
	rest_framework/tests/fields.py
Moved test model into closer to the testcase
Merge pull request #853 from pyriku/725-blank-choice-dash

Adds BLANK_CHOICE_DASH as a choice item