Add test demonstrating how a valid token sig can be marked invalid.
Add test demonstrating how a valid token sig can be marked invalid.

The way elixir (poison?) encodes the JWT map has an effect on the order,
potentially leading to a valid signature being marked as invalid. This
can happen when working with JWTs generated outside of Joken/Elixir.

cs-victor-nascimento/joken,potatosalad/joken