Codeception issue #1510 - site-root relative URL handling

Changing Guzzle::getAbsoluteUri to handle site-root relative URLs.  This changes how PhpBrowser handles relative URLs everywhere, including amOnPage, click(), submitForm, etc...

@DavertMik please review, this may be more appropriate - but it changes how URLs are handled everywhere.  I've also added to the test with links and 'amOnPage' to demonstrate the changes and test.

Thanks,

Zaahid