fix getJWKByKid return types

[satanio]

readPublicKeyCache returns the result of json_decode -> thats an array of strings.

[cla-assistant]

All committers have signed the CLA.

[Ynhockey]

Thanks for the contrib. I think it's probably not a good idea that it can return either one, must have been an oversight on my part with the latest upgrade. I will check it sometime soon and try to understand what the correct behavior should be.

[satanio]

any update? I understand it is not of utmost importance, but a year passed :D