Allow Session#visit to visit absolute URLs with no scheme.

Hello,

Looking at [the code](https://github.com/jnicklas/capybara/blob/master/lib/capybara/session.rb#L209), it looks like Capybara considers any URL without scheme to be relative.
Actually, an absolute URL can have an empty scheme: when encountering `//example.com`, browsers will use the same scheme as the current page (thus https://example.com if you're visiting an HTTPS page, or http://example.com otherwise).

Please find my suggestions in this pull request.

Regards,

David