Add test cases for association object type many to many relations

* Add models defining m2m relation
* Test posting a new m2m relation
* Test patching an object without any existing m2m relations
* Test patching an object with an existing m2m relation
* Test patching an object to remove a m2m relation

Note: An association object many to many relation allows us to
have a normal many to many relation that holds extra data in the
helper table.