Allow multiple as host_url (#465)

app/helpers/sessions_helper.rb

@@ -65,8 +65,12 @@ module SessionsHelper
 
   def parse_user_domain(hostname)
     return hostname.split('.').first unless Rails.configuration.url_host
-    return '' unless hostname.include?(Rails.configuration.url_host)
-    hostname.chomp(Rails.configuration.url_host).chomp('.')
+    Rails.configuration.url_host.split(',').each do |url_host|
+      if hostname.include?(url_host)
+        return hostname.chomp(url_host).chomp('.')
+      end
+    end
+    ''
   end
 
   def omniauth_options(env)