Make absolute uri test more general

This test had hardcoded values which may vary for different apps and while this specs are integrated in custom driver's test suite like capybara-mechanize, it fails becouse of that (https://travis-ci.org/jeroenvandijk/capybara-mechanize/jobs/17002693#L153)

Problem occurred when I was trying to bump up capybara version for capybara-mechanize gem where app have different host (`www.local.com` not `localhost`) and don't have `/foo/bar` endpoint.