Fix for Codeception issue #1510 - site-root relative paths

Using a site-root relative path for a form's action fails if PhpBrowser
is configured with a 'url' basepath with a subdir (e.g.
localhost/subdir)
You're right, it isn't pretty - but if I use parse_url, I'd have to worry about rebuilding the URL afterwards and http_build_url is part of PECL, not standard.

That would mean I'd have to concatenate the parts returned from parse_url and worry about protocol, host, ports, username, etc... and I feel like getFormUrl isn't the place to do that.

Can you think of another way, not reliant on parse_url?  Or perhaps there's an api being used somewhere else in Codeception?