Fixed token revoking call to IdP.

[axl8713]

The revoking call in the logout procedure was wrongly sending the access_token instead of the required id_token.
In fact, GeoStore logs showed this:

it.geosolutions.geostore.services.rest.security.oauth2.OAuth2SessionServiceDelegate::logRevokeErrors:1025 - Error while revoking authorization. Error is:
  org.springframework.web.client.HttpClientErrorException$BadRequest: 400 Bad Request: "{"error":"unsupported_token_type","error_description":"Unsupported token type"}"

These changes fix the issue, adding also propert testing.