MySQL backend schema editor skip_default method requires update for MySQL 5.7

[conor]

Currently this method, ​django.db.backends.mysql.schema.DatabaseSchemaEditor.skip_default, checks for a certain group of field types (texts and blobs) to determine whether defaults are applicable at the database level. Since MySQL 5.7, there is one more which I believe needs to be added to this list - the new 'json' type. I discovered this while working with the ​django-mysql module. In that module a default of dict is set at the Django model level but needs to be ignored for database operations.
As far as I can see it should be a pretty straight forward fix as I can't see why it would interfere with the functionality for older versions of MySQL.

[Adam Johnson]

PR: ​https://github.com/django/django/pull/6997

[Tim Graham <timograham@…>]

In 4b1c9708:

Fixed #26966 -- Made MySQL backend skip defaults for JSON columns

Thanks mcgeeco for reporting, and claudep and timgraham for review.