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)
Never mind - you've answered that already!

I left a note in #1510 as well.  In the meantime I will implement your proposed change for this but I recommend holding off on merging until you get a chance to review that as well :)