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.
Sidenote on tests: Um, ok, so the tests are in a kind of sorry (and complicated) state. The test @groups don't mean anything at least w/ regard to testsuites, instead the directories in PHPUnit are used to differentiate tests. Right now, Core tests do not use the database, and I think should not use the network (there are 2 existing tests that do). Integration tests run w/ a database and they extend either DatabaseTestCase or IntegrationTestCase. They can test more than API methods, and they are meant to (eg, ImportLogsTest), calling API methods is just the convention used to test (been around since before I started committing). I think they will be rewritten eventually, hopefully soon.

As for this test, I'm just wary of using curl when we've had discussions in the past to move away from using the network in Core tests. It would be nice if the re-routing logic could be tested by itself, but perhaps this is not yet possible w/ Piwik's infrastructure. It looks like the regex logic is being tested, so maybe it might be a good idea to move the logic that parses & transforms URLs to another class and then unit test that class?

@tsteur could probably say more regarding tests.