Make Devise.mappings work with lazy loaded routes.

When `Devise.mappings` is called it expects that routes are loaded because mappings is build
by `devise_for` call.

Author: nashby

lib/devise.rb

@@ -277,6 +277,12 @@ module Test
   mattr_reader :mappings
   @@mappings = {}
 
+  def self.mappings
+    Rails.application.routes_reloader.try(:execute_unless_loaded)
+
+    @@mappings
+  end
+
   # OmniAuth configurations.
   mattr_reader :omniauth_configs
   @@omniauth_configs = {}