Handle URLs starting with index.php correctly

If the URL "index.php" was given, it was converted into "http://http://<servername>/index.php", and an URL starting with "index.php" (e.g. "index.php/sample-sites") was converted into (in this example) "http://index.php/sample-sites". Now "index.php" and e.g. "index.php/sample-sites" are treated identically, they are converted into "http://<servername>/index.php" resp. "http://<servername>/index.php/sample-sites".
Merge pull request #4321 from jo-sf/patch-10

Handle URLs starting with index.php correctly