Enforce types of the directly supported claims when they are set as Objects

[sberyozkin]

Fixes #391

I've been thinking for a while what exactly would be the right course of action when someone types .claim("sub", 1) and decided that instead of silently converting to String and causing some potentially difficult to diagnose problems it would be better to enforce the correct types for only those claims which are well-known (ex JWT and Mp JWT specs) but more importantly those which are directly (have the dedicated setters) supported by this API - this will also ensure the consistent handling of such claims, whether they set in a typed way or as Objects.

A few simple claims must be Strings, aud or groups - Strings or Collections of Strings, iat and exp - long or Instant.

Also improved the documentation. And added claim setters accepting Claims as a claim name

[sberyozkin]

@radcortez @MikeEdgar thanks