Fix confusion of received response message

This is a follow up for commit 33fb966 ("Remove sender/message_id
pair after response received in resolv", 2020-09-11).

As the @senders instance variable is also used for tracking transaction
ID allocation, simply removing an entry without releasing the ID would
eventually deplete the ID space and cause
Resolv::DNS.allocate_request_id to hang.

It seems the intention of the code was to check that the received DNS
message is actually the response for the question made within the method
earlier. Let's have it actually do so.

[Bug #12838] https://bugs.ruby-lang.org/issues/12838
[Bug #17748] https://bugs.ruby-lang.org/issues/17748

Author: rhenium

lib/resolv.rb

@@ -696,17 +696,17 @@ def request(sender, tout)
           rescue DecodeError
             next # broken DNS message ignored
           end
-          if s = sender_for(from, msg)
+          if sender == sender_for(from, msg)
             break
           else
             # unexpected DNS message ignored
           end
         end
-        return msg, s.data
+        return msg, sender.data
       end
 
       def sender_for(addr, msg)
-        @senders.delete([addr,msg.id])
+        @senders[[addr,msg.id]]
       end
 
       def close