use .seek(0) instead of reset() for compatibility

Since I don't find docs for the .reset() method, I lookup up the source
code, and it should work just the same as .seek(0).

For reference:
8eeb7e9122/Modules/cStringIO.c (L281)

scrapy/core/downloader/handlers/ftp.py

@@ -54,7 +54,7 @@ class ReceivedDataProtocol(Protocol):
         return self.__filename
 
     def close(self):
-        self.body.close() if self.filename else self.body.reset()
+        self.body.close() if self.filename else self.body.seek(0)
 
 _CODE_RE = re.compile("\d+")
 class FTPDownloadHandler(object):

scrapy/tests/test_contrib_exporter.py

@@ -132,7 +132,7 @@ class PickleItemExporterTest(BaseItemExporterTest):
         ie.export_item(i1)
         ie.export_item(i2)
         ie.finish_exporting()
-        f.reset()
+        f.seek(0)
         self.assertEqual(pickle.load(f), i1)
         self.assertEqual(pickle.load(f), i2)