Test and quick fix for #1257

Addresses #1257. 

Fields defined explicitly with `required=False` will be in the list of exclusions from validation. 

I did not address [the idea for a list of non-required fields](https://github.com/tomchristie/django-rest-framework/issues/1257#issuecomment-29737578). 

0: Adding (and documenting) an `optional_fields` list that simply acted as another sub-clause to the `if field_name in exclusions...` check would be simple enough. 

However:

1. Do we want extra API here?
2. Would we need to do something clever if fields appeared in such a list but were also explicitly defined etc?