Previously, when parsing claim values, we used `json.UnsupportedTypeError` to denote if a claim string value is not of the correct type. However, this could lead to panics if a nil value is present and the `Error` function of the `json.UnsupportedTypeError` is called, which does not check for nil types.
Instead, we just now use `ErrInvalidType` similar to the map claims.
Fixes#315
This PR aims at implementing compliance to RFC7519, as documented in #11 without breaking the public API. It creates a new struct `RegisteredClaims` and deprecates (but not removes) the `StandardClaims`. It introduces a new type `NumericDate`, which represents a JSON numeric date value as specified in the RFC. This allows us to handle float as well as int-based time fields in `aud`, `exp` and `nbf`. Additionally, it introduces the type `StringArray`, which is basically a wrapper around `[]string` to deal with the oddities of the JWT `aud` field.
Christian Banse