OPENIG-418 (CR-5596) Accepts OAuth Access Token Response with 'expires_in' field expressed as String

Recent Google OAuth 2.0 update is breaking the OAuth 2.0 Client Filter by having
the `expires_in` attribute of the access token response expressed as a String
instead of a JSON Number.

We're now doing the adaptation in OpenIG and deal with the String case by hand.

git-svn-id: https://svn.forgerock.org/openig/trunk@801 dbb9e58e-28e6-4ce0-90e8-f11d9605b710