support 2.2 new related manager behavior

[MichaelKim0407]

Pre-2.2, some related manager methods are disabled for m2m relations with a through model.

2.1 doc:

Also, if you are using an intermediate model for a many-to-many relationship, then the add(), create(), remove(), and set() methods are disabled.

This has been changed in 2.2 with a through_defaults argument.