Fix visiting urls with non-http(s) schemes

Fixes #1336 
It is documented in the Calling remote servers section of the readme  -   "Normally Capybara expects to be testing an in-process Rack application, but you can also use it to talk to a web server running anywhere on the internet, by setting app_host:"    ....       "By default Capybara will try to boot a rack application automatically. You might want to switch off Capybara's rack server if you are running against a remote application:  Capybara.run_server = false"

ie -- app_host has precendence -- and you may want to turn off server if you're using it (or may not - either way app_host has precendence)