Fixed issue with SimpleRouter::error not firing within group (issue: #551).

- Fixed variable incorrect variable reference in `InputItem` class.
- Added new `Router::addExceptionHandler` method.
- Added parameter types in `Url` class.
- Fixed phpdoc parameter-type for `Request::getHeader`.

src/Pecee/SimpleRouter/SimpleRouter.php

@@ -427,9 +427,7 @@ class SimpleRouter
     {
         $callbackHandler = new CallbackExceptionHandler($callback);
 
-        static::router()->addRoute(
-            (new RouteGroup())->addExceptionHandler($callbackHandler)
-        );
+        static::router()->addExceptionHandler($callbackHandler);
 
         return $callbackHandler;
     }