Add unit test cases to parse JWT tokens signed with ECDSA keys

[sebastien-rosset]

In parse_test.go:

1. add test case where the JWT token is signed with a ECDSA private key and "EC256" is in the list of accepted algorithms.
2. add test case where the JWT token is signed with a ECDSA private key but "EC256" is NOT in the list of accepted algorithms.
3. Add validation for Token.Valid and Token.Signature in the unit tests.

[lggomez]

LGTM, nice to have in the test coverage

[oxisto]

LGTM