Improve handling of JWK parameters with the wrong type

[Brian Campbell]

Currently a runtime ClassCastException is thrown when processing a JWK that has a parameter with the wrong value type. I.e. a 'kty' with a numeric value rather than a string.

A checked JoseException with a more meaningful message would be preferable and more consistent with how other issues, like missing a required parameter, are handled when parsing/processing JWKs.

[Brian Campbell]

50689e0 -> as fix for #81 a JWK parameter that is expected to be a String but is a different type will now result in a JoseException with a somewhat descriptive message rather than a ClassCastException

[Brian Campbell]

done in v 0.5.4