Encode any JSON key to string, closes #14305

[josevalim]

cc @michalmuskala

[michalmuskala]

The main issue with the Jason API around this was error handling for the tuple-returning variant. In particular initially I didn't rescue the String.Chars protocol not implemented errors, and this was always left like that for fear of breaking compatibility. Perhaps the Elixir API can do this well.
That said, I see it doesn't seem to be rescuing the protocol-not-implemented errors for JSON.Encode either, so perhaps that's actually the better behaviour.

[josevalim]

We don't provide the tuple variant exactly because of issues like these. :) So I will go ahead, merge this, and ship a new version! Thank you!

[whatyouhide]

This is nice! Maybe some comments in the code for why we do this would help but code-wise it looks spot on

[josevalim]

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