omniauth routes fix, see devise issue #600

lib/devise/omniauth/url_helpers.rb

@@ -7,7 +7,7 @@ module Devise
         class_eval <<-URL_HELPERS, __FILE__, __LINE__ + 1
           def #{mapping.name}_omniauth_authorize_path(provider)
             if Devise.omniauth_configs[provider.to_sym]
-              "#{mapping.fullpath}/auth/\#{provider}"
+              "/#{mapping.path}/auth/\#{provider}"
             else
               raise ArgumentError, "Could not find omniauth provider \#{provider.inspect}"
             end

lib/devise/rails/routes.rb

@@ -239,7 +239,7 @@ module ActionDispatch::Routing
       end
 
       def devise_omniauth_callback(mapping, controllers) #:nodoc:
-        path_prefix = "#{mapping.fullpath}/auth"
+        path_prefix = "/#{mapping.path}/auth"
 
         if ::OmniAuth.config.path_prefix && ::OmniAuth.config.path_prefix != path_prefix
           warn "[DEVISE] You can only add :omniauthable behavior to one model."