Allow MultipartStreamBuilder subclass to manipulate data

* Change property $data visibility from "private" to "protected".
  Leaving more flexibilities for subclassing.

Author: yookoala

src/MultipartStreamBuilder.php

@@ -36,7 +36,7 @@ class MultipartStreamBuilder
     /**
      * @var array Element where each Element is an array with keys ['contents', 'headers', 'filename']
      */
-    private $data = [];
+    protected $data = [];
 
     /**
      * @param HttplugStreamFactory|StreamFactoryInterface|null $streamFactory