Accessor method for Environment::$_detector. If $_detector is unset, returns the default
detector built into the class. For more information on setting and using $_detector, see
the documentation for Environment::is(). The _detector() method is called at the
beginning of the application's life-cycle, when Environment::set() is passed either an
instance of a Request object, or the $_SERVER or $_ENV array. This object (or array)
is then passed onto $_detector, which returns the correct environment.