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/Http/Input/InputItem.php

@@ -97,7 +97,7 @@ class InputItem implements ArrayAccess, IInputItem, IteratorAggregate
 
     public function offsetUnset($offset): void
     {
-        unset($this->data[$offset]);
+        unset($this->value[$offset]);
     }
 
     public function __toString(): string