On MySQL, text, blob, json, and the short/medium/long

varieties of the first two are forbidden from having default values.
Django Evolution would try to apply these anyway when adding a non-NULL

field or changing a field to be non-NULL (and in fact validated that

such a value would be provided when setting to non-NULL). This would

prevent these types of fields from being added unless they were set to

allow NULL.
Now, the evolution op backends can override a

get_field_type_allows_default() method, which can check a field to

determine if default values should be allowed. This is used when

building a column schema definition, ensuring that any DEFAULT-related

SQL is omitted if not allowed for the field.

Unit tests pass for all supported database types on all supported

versions of Django.