Signature value should not be converted to hex string

[paweldomas]

According to the rfc7515 the result of hash function(signature) should not be converted to hash string, but only BASE64URL encoded:

Computing the HMAC of the JWS Signing Input ASCII(BASE64URL(UTF8(JWS
Protected Header)) || '.' || BASE64URL(JWS Payload)) with the HMAC
SHA-256 algorithm using the key specified in Appendix A.1 and
base64url-encoding the result yields this BASE64URL(JWS Signature)
value:

dBjftJeZ4CVP-mB92K27uhbUJU1p1r_wW1gFWFOEjXk

Currently the library is not interoperable with other JWT libraries(see results with debugger at https://jwt.io). The tests should be probably changed to compare raw values instead of working with the values computed by the library's encode function:

https://github.com/Olivine-Labs/lua-jwt/blob/master/spec/jwt_spec.lua#L40