Fixes #6156 Redirect /index.php/.whatever?... to /index.php?...

See #6156. `/index.php/.html?...` URLs can be interpreted by old browsers using the extension instead of the Content-Type header. This redirection should prevent that.

The part that does the redirection is in the `FrontController` (just before the dispatching). If there is a better place for this (to avoid crowding up the front controller) let me know.

I have also added a simple PHPUnit test for this. I ended up using `curl` manually because the `Http` class doesn't return the HTTP response headers. If I missed how to do that properly with `Http`, let me know too.
In a branch I am working on I already moved some of them to Integration/System tests. If the network request goes to Piwik itself again it would be rather a system test then integration test. I tried to work on some kind of definition for this here but need to add more information: https://github.com/piwik/piwik/blob/5940_testRenamingAndCorrectFolders/tests/README.md#how-to-differentiate-between-unit-integration-or-system-tests