Set typ field in JSONWebToken::RSAToken

ruby-jwt v2.0 removed the `typ` header in
https://github.com/jwt/ruby-jwt/commit/cc41d53e00f8cbf015271b53f5ad761bd6ac2312.

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

lib/json_web_token/rsa_token.rb

@@ -11,7 +11,8 @@ module JSONWebToken
 
     def encoded
       headers = {
-        kid: kid
+        kid: kid,
+        typ: 'JWT'
       }
       JWT.encode(payload, key, 'RS256', headers)
     end