revoke_uri return a google url even if used against another OAuth endpoint provider #323

Thanks so much for making this work with other OAuth providers, but I noticed 
when I sent a request to one of those external providers I get back:

{
    "client_secret": "client_secret_goes_here",
    "revoke_uri": "https://accounts.google.com/o/oauth2/revoke",
    "access_token": "access_token_goes_here",
    "id_token": null,
    "refresh_token": null,
    "client_id": "client_id_goes_here",
    "token_response": {
        "token_type": "bearer",
        "access_token": "access_token_goes_here"
    },
    "_class": "OAuth2Credentials",
    "_module": "oauth2client.client",
    "token_expiry": null,
    "token_uri": "https://app.someotherapp.com/oauth/token",
    "user_agent": null,
    "invalid": false
}

I think the revoke_uri should either be blank or something if the provider gave 
one back,  but as it stands I doubt you're taking revoke requests for other 
vendors :-)? Thanks!