fixing getRawPathInfo() once more

fixes #6050
#6035 was implemented by me under the assumption that the script name (e.g. index.php, public.php ...) is required in the url - this was a misunderstanding as #6050 has shown.

Previous to the patches in #6035 the request to the owncloud url without the index.php was silently failing and redirected to index.php - accidentally correct. :wink:

Within the unit tests you will find a lot of examples on how this function works now.