Issue 747 lazy strings serialized

I made the change you proposed on https://groups.google.com/d/msg/django-rest-framework/6qrr4iVgIrw/1Acsp542m9YJ about the issue 747

I also added a test to ensure the string are being converted into unicode.

Do you think we need a test to ensure they are correctly translated? force_text should be already tested.