Use TypedMultipleChoiceField for ArrayField with choices.

This pull request implements better default formfield support for `ArrayField` with `choices` set. The appropriate default form field for that situation is `TypedMultipleChoiceField` (rather than Django's default of `TypedChoiceField`), and it is inappropriate to add a blank choice (the equivalent for an array is simply to not select any choices). This pull request implements those two changes (such that they remain overridable by the user), and also sets the type-coercion function for the form field to match the `ArrayField` type-coercion function.