Treat responses without content-length or Transfer-Encoding as good responses

There is not a way to determine if responses without Content-Length or Transfer-Encoding are complete, this change treat them as good responses but flags them as "partial".
This is backout change only for this functionality of 3c64a989

scrapy/core/downloader/handlers/http11.py

@@ -148,5 +148,7 @@ class _ResponseReader(protocol.Protocol):
         body = self._bodybuf.getvalue()
         if reason.check(ResponseDone):
             self._finished.callback((self._txresponse, body, None))
+        elif reason.check(PotentialDataLoss):
+            self._finished.callback((self._txresponse, body, ['partial']))
         else:
             self._finished.errback(reason)