fix(protocols): previous OAuth2 config parameter rename broke functio…

…nality

This restores the previous config parameter name of `client_id` for
OAuth2 providers (e.g. google).

[closes #368]

protocols/OAuth2.js

@@ -127,7 +127,7 @@ function authorizationRequest (req, options) {
   var config = this.client
   var url = URL.parse(endpoints.authorize.url)
   var responseType = 'code'
-  var clientId = config.clientId
+  var clientId = config.client_id
   var redirectUri = provider.redirect_uri
   var state = options.state