Fix creating an undocumented dynamic property

https://wiki.php.net/rfc/deprecate_dynamic_properties
causes this to emit an E_DEPRECATION notice.

A private property was chosen because:
- This was undocumented and I assume unintentional.
- This will not conflict with any properties of the same name and
  different types (or readonly properties) in subclasses.

lib/Dispatcher.php

@@ -39,6 +39,10 @@ class Dispatcher
      * @var \phpDocumentor\Reflection\Types\ContextFactory
      */
     private $contextFactory;
+    /**
+     * @var JsonMapper
+     */
+    private $mapper;
 
     /**
      * @param object $target    The target object that should receive the method calls