Fix google:clientSecret Regex

lib/form/admin/google.js

@@ -5,6 +5,6 @@ var form = require('express-form')
 
 module.exports = form(
   field('settingForm[google:clientId]').trim().is(/^[\d\.a-z\-\.]+$/),
-  field('settingForm[google:clientSecret]').trim().is(/^[\da-zA-Z\-]+$/)
+  field('settingForm[google:clientSecret]').trim().is(/^[\da-zA-Z\-_]+$/)
 );