Test MultiChoiceField.validate when valid.
Test MultiChoiceField.validate when valid.

Also, I realized that the `choices` kwarg needs to be a nested list.

frankier/mezzanine,molokov/mezzanine,stephenmcd/mezzanine,frankier/mezzanine,ryneeverett/mezzanine,jerivas/mezzanine,christianwgd/mezzanine,frankier/mezzanine,wbtuomela/mezzanine,stephenmcd/mezzanine,dsanders11/mezzanine,ryneeverett/mezzanine,jerivas/mezzanine,dsanders11/mezzanine,wbtuomela/mezzanine,wbtuomela/mezzanine,dsanders11/mezzanine,readevalprint/mezzanine,readevalprint/mezzanine,christianwgd/mezzanine,stephenmcd/mezzanine,ryneeverett/mezzanine,readevalprint/mezzanine,christianwgd/mezzanine,molokov/mezzanine,molokov/mezzanine,jerivas/mezzanine