don't invoke response block more than once due to retry

If a socket error occurs while performing a streaming download via
the response block provided to transport_request, avoid calling
the response block again as this would result in duplicate data
received by the client.

fixes #86

lib/net/http.rb

@@ -2350,7 +2350,10 @@ def transport_request(req)
           res
         }
         res.reading_body(@socket, req.response_body_permitted?) {
-          yield res if block_given?
+          if block_given?
+            count = max_retries # Don't restart in the middle of a download
+            yield res
+          end
         }
       rescue Net::OpenTimeout
         raise