Allow to post existing relationships

[ArneZsng]

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When adding a relationship that already exists or multiple relationships of which at least one already exists, the response is a 400 Bad Request and the relationships are not updated/created. In our specific case we expected the API to behave differently and if I am not misinterpreting http://jsonapi.org, the expected response is actually a different one.

To quote http://jsonapi.org/ (http://jsonapi.org/format/#crud-updating-to-many-relationships):

If all of the specified resources can be added to, or are already present in, the relationship then the server MUST return a successful response.
Note: This approach ensures that a request is successful if the server’s state matches the requested state, and helps avoid pointless race conditions caused by multiple clients making the same changes to a relationship.

Fixes #1168

Reviewer Checklist:

• Maintains compliance with JSON:API
• Adequate test coverage exists to prevent regressions

[lgebhardt]

@ArneZsng Thanks!