Fixed #20895 -- Made check management command warn if a BooleanField doe...

...s not have a default value

Thanks to Collin Anderson for the suggestion.

This is my first code patch for Django, so any feedback is welcome, I'm happy to make any changes that are required. 

In particular, I wasn't sure whether monkey patching the field default was the best approach, but I couldn't see the best way to check the model in one particular test but not the others.