Added $mapperClass property to AdapterAbstract

This was missing in the abstract, but used in an implementation

Author: Tom Van Herreweghe

lib/PHPExif/Adapter/AdapterAbstract.php

@@ -39,6 +39,11 @@ abstract class AdapterAbstract implements AdapterInterface
      */
     protected $hydrator;
 
+    /**
+     * @var string
+     */
+    protected $mapperClass = '';
+
     /**
      * Class constructor
      *