Make `Capybara.server_host` working properly

Setting `server_host` was added by 2dca77060ab33fe35ce3996f9721657a8d604ca3 but it doesn't work properly.
Webrick was still binding host by default it's `0.0.0.0`, `host` in `server.rb` related just to request `Net::HTTP.start`.
So the test I commented doesn't test that Webrick binded proper host, actually I have no clue how to test that... and just commented that one to pay you attention to it.