Add scope (OPTIONAL) into Token\Response::allowedProperties

Access Token Response might contain 'scope'.
So, we have to add 'scope' into $allowedProperties.

4.4.3.  Access Token Response
https://tools.ietf.org/html/rfc6749#section-4.4.3

5.1.  Successful Response
https://tools.ietf.org/html/rfc6749#section-5.1