Add error code under meta in serialization

[Startouf]

Serialize ActiveModel "error symbol"s under the meta key in the jsonapi payload as suggested in #11

[Startouf]

Hey @richmolj since we needed #11 in our app I took the freedom to start a PR on one of the suggestions in the issue, but It's probably better to settle on something in the issue and then come back to this PR or something different and add the tests.

(Tbh this PR is actually more convenient for our app since I can mention a pending PR on my Gemfile)

This implementation is actually quite coupled to the way ActiveModel does things... but it's ok ?

[richmolj]

Hey @Startouf thanks for this issue and PR - well described and I agree with your solution. Could we add a test for a PORO including ActiveModel::Validations? As long as that test passed this would be 💯 👏

[richmolj]

@Startouf as luck would have it this same use case just came up for me. I'll be refactoring the code and tests tomorrow and will make sure to support this use case 👍

[richmolj]

@Startouf could you confirm #13 works for you and if so close the PR?

[Startouf]

Hey I'm going to test that right away, sorry didn't get the notifications.

[Startouf]

Solved by #13