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.
I don't think we should be using curl within unit tests (ie, where directory is PHPUnit/Core, the @group's lie, don't listen to them). Can you test the relevant logic w/o creating HTTP requests?