Allow a list of strings as header value in JWT::encode()

[herndlm]

Fixes #593

In our case we need to be able to set the crit claim for a third party webservice. Found more info about crit here: https://mojoauth.com/glossary/jwt-critical/

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[herndlm]

I don't have a google account associated with my email and don't want to create one. I'm ok with the CLA though. anyway, anybody feel free to take over this fix if possible. I also don't care if this is copied from me without mentioning me (e.g. @hultberg)..

that's how you make things hard in OSS with weird processes 😓