Update many-to-many through PATCH on resource not working

[maurei]

Exactly same part of the spec as #492 but for many-to-many instead of one-to-many.

Currently creating relations in the simplest scenario works, as clarified by this test

More complex scenarios will fail. Eg, consider and article with tag_1

• sending a patch with a new relationship to tag_2 (not including tag_1) results in having both. No complete replacement was performed
• Similarly, when including both tag_1 and tag_2, it will try to recreate a relationship to tag_1, which will break