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.
Totally agree with you, but I was wary of doing that in the same process because that would mean changing global state just for the test (like `$_SERVER`) and having the FrontController set HTTP headers (again, global state). If several tests did the same thing, that would completely mess up.

I put it in the "Core" tests because it was consistent with the other tests around (many do HTTP requests). Integration tests seem to all be built against `Piwik\Tests\IntegrationTestCase` which is done to test API methods. But I would totally be OK moving that in integration tests, that would be the right place, I was just afraid it wouldn't be consistent.