Add typ header back into JWT payload

ruby-jwt v2.0 removed the `typ` header in
cc41d53e00.

To ensure tokens don't get marked invalid during an upgrade,
add it back to ensure backwards compatibility.

lib/json_web_token/hmac_token.rb

@@ -18,7 +18,7 @@ module JSONWebToken
     end
 
     def encoded
-      JWT.encode(payload, secret, JWT_ALGORITHM)
+      JWT.encode(payload, secret, JWT_ALGORITHM, { typ: 'JWT' })
     end
 
     private