Ensure generated entity classes include association fields.

Because composer caches whether or not a class exists, we cannot simply
reload the table class once it has been created. Instead if bake is
using a base Table instance, we can add the associations that would be
present into the table. This allows the entity's `$_accessible`
attribute to be correctly generated.

Refs #3715
Merge pull request #3719 from cakephp/3.0-bake-entity-associations

Ensure generated entity classes include association fields.