Enable side-loading of has_many, through associated resources.

This PR updates the restpack serializer to properly handle has_many through associations. I added tests to demonstrate that it was previously broken but is now fixed.
@GavinJoyce this will be a hash when the where condition needs to chain through the join table, [as seen here](https://github.com/RestPack/restpack_serializer/pull/80/files#diff-cb21b7a11f5d0c43a98d8bd61d6361e9R92).