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/Router.php

@@ -944,4 +944,10 @@ class Router
         return $this;
     }
 
+    public function addExceptionHandler(IExceptionHandler $handler): self
+    {
+        $this->exceptionHandlers[] = $handler;
+        return $this;
+    }
+
 }