Allow configure Faye's monitoring when using mideleware indirectly

When you use Rack middleware indirectly, with either use/insert_beforet etc. in config.ru or Rails configuration block, you have no way to get the instance and configure monitoring on it. WIth these changes, you can do it straight from config.ru by passing a block.

Please note the change I had to make (assertion -> stub) in specs file to make my test work. If you think this is a wrong approach, I'm happy to change it.