Compliance with published RFC7519

[oxisto]

When the library was initially started, RFC7519 did not exist - only its drafts. This has lead to some shortcomings over the years where the implementation, especially of the claims, diverged from the now published RFC.

For example, the draft at the time of the writing of the claims struct actually stated that iat, exp and so on must be an IntDate, which did not include the possibility of non-integer date representations. It got then later changed to a NumericDate format in draft version 26, which - as @dunglas correctly pointed out - included also float representations (also strings).

The aim would be to be as backwards compatible as possible, e.g. with the introduction of a new struct as suggested by @mfridman, which I will name for the lack of a better name jwt.RFC7159Claims for now.

This also was the original problem behind the audience mixup in #6

Points to consider (and possibly replaced with a new struct or function):

• The fields ExpiresAt, IssuedAt, NotBefore in jwt.StandardClaims are int64 instead of a (to be defined) NumericDate type. Probably solvable with a new struct
• The functions VerifyIssuedAt, VerifyNotBefore, VerifyIssuedAt of jwt.StandardClaims only compare against an int64 instead of the NumericDate. Also probably solvable with a new struct
• The functions VerifyIssuedAt, VerifyNotBefore, VerifyIssuedAt of jwt.MapClaims only compare against an int64 instead of the NumericDate. Trickier. I do not want to touch jwt.MapClaims too much. Probably the introduction of a new set of functions can be made and the old ones set to deprecated.
• Audience of jwt.StandardClaims should be a []string instead of string. Could also be part of the new struct

Anything else that comes to mind?

At some point, it would also probably a good idea to check, whether RFC8725 is also properly reflected in the implementation.

Originally posted by @oxisto in #9 (comment)

[qbiqing]

Audience of jwt.StandardClaims should be a []string instead of string. Could also be part of the new struct

This would be useful to me.

[oxisto]

Audience of jwt.StandardClaims should be a []string instead of string. Could also be part of the new struct

This would be useful to me.

Yeah, I created a new type ClaimStrings which can be either serialized/deserialized as a []string or string. The PR is ready (#15), just waiting for an approval by another maintainer before I can merge this in.