Using Capybara.app_host with set_url

It has happened to me when testing non rails apps using capybara and site_prism, that the base url is repeated on each page I used. So this pull request affects the set_url method, checking if the Capybara.app_host has been set and prepends it to the url passed to set_url method.

If the url passed starts with (http|https):// , then it assumes that Capybara.app_host should be ignored.