tests: add jsonapi schema validation where appropriate

[mfn]

Fixes #25

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Continuing #25 (comment) , here an implementation how I envisioned it.

• adds a new trait with a new assertion
• apply the assertion where appropriate

Now, before continuing here, a few things:

• the branch fails because the error responses are currently not according to the spec
  • status and core must be of type string => Fixed in errors: ensure response matches jsonapi spec #28
  • stacktrace is not allowed in error directly; the spec provides meta where this can be put
• the response tests also use structures violating the standard, they would need to be changed

Do you like this direction?

I'm willing to take care and enhance this PR once the other PRs are OK for you too. It gets a bit messy due all the inter-dependant changes :-)

[egeriis]

This is really awesome. It would be cool if this could be expanded beyond the error response. Do you have any ideas for that?

[mfn]

This is really awesome. It would be cool if this could be expanded beyond the error response. Do you have any ideas for that?

The idea would be to rewrite some of the tests in the first place to be jsonapi schema 1.0 compliant. I don't necessary plan to do it, however I think the PR itself is already a benefit to the tests on its own.

[egeriis]

Great. Just wanted to hear if you planned on adding to the PR. Thank you for all the contributions, that's deeply appreciated.

[egeriis]

Since you've been so deep into the lib already, I'd be thrilled to hear your thoughts regarding #32