Set host for ActionMailer instances.

Fixes #256.

I'm not happy with this, but at least it works. For now.

lib/draper/view_context.rb

@@ -26,6 +26,11 @@ module Draper
 
     def self.build_view_context
       current_controller.view_context.tap do |context|
+        context.instance_eval do
+          def url_options
+            ActionMailer::Base.default_url_options
+          end
+        end unless context.request
         if defined?(ActionController::TestRequest)
           context.controller.request ||= ActionController::TestRequest.new
           context.request            ||= context.controller.request