Incorporate string array in aud field from the original v4 branch

[dcormier]

The original repo has a pretty complete v4 release that I'm using (docs).

As I recall, I needed v4 because of this change:

Added new type, ClaimStrings, which will correctly handle properties such as aud that can be either an array of strings or a single string. ClaimStrings is an alias to []string, but with custom decoding behavior. This means all the built in aud validation behavior now expects []string instead of string. This was a common request.

It would be nice to see that picked up and moved forward, as well.

[oxisto]

I have tried to put that functionality in #15. Could you have a look whether this fulfills your use case?

[dcormier]

It looks like that probably would handle that my use case.

[dunglas]

This is currently a BC break. When using StandardClaims with an aud field containing a string, this error is now thrown: unable to parse JWT: json: cannot unmarshal array into Go struct field claims.aud of type string.

Using the new RegisteredClaims introduced in #15 works properly, but even with this patch the BC break is still present of the deprecated StandardClaims struct.