media: use deferred of request to allow adding callback from get_media_requests

--HG--
extra : convert_revision : svn%3Ab85faa78-f9eb-468e-a121-7cced6da292c%40249

scrapy/trunk/scrapy/contrib/pipeline/media.py

@@ -62,7 +62,7 @@ class MediaPipeline(object):
         if fp not in info.downloading:
             self._download(request, info, fp)
 
-        wad = defer.Deferred()
+        wad = request.deferred or defer.Deferred()
         waiting = info.waiting.setdefault(fp, []).append(wad)
         return wad