Updated isJWT so it's conforming to specs

[ItalyPaleAle]

Updated the regular expression for isJWT so it's conforming to the specs.

#609 contained a regular expression where the signature part was required. However, RFC 7519 states:

To support use cases in which the JWT content is secured by a means
other than a signature and/or encryption contained within the JWT
(such as a signature on a data structure containing the JWT), JWTs
MAY also be created without a signature or encryption. An Unsecured
JWT is a JWS using the "alg" Header Parameter value "none" and with
the empty string for its JWS Signature value, as defined in the JWA
specification [JWA]; it is an Unsecured JWS with the JWT Claims Set
as its JWS Payload.

Note that the code in the PR doesn't check if "alg" is actually "none" when the signature is missing, as it doesn't decode the JWT... I'm happy to implement that if you think I should, however.

I have also added some stricter checks to ensure that the base64 tokens are valid.

[chriso]

Thanks!