Silently ignore answers to packets that aren't ignored on closed connection

Commits like 7eb7cf1b
("Minimize the amount of work during tpc_finish") dropped what was done in
commit 07b48079
("Ignore some requests, based on connection state") to protect request handlers
when they respond.

This commit fixes this in a generic way.

neo/lib/connection.py

@@ -576,10 +576,12 @@ class Connection(BaseConnection):
 
     def answer(self, packet):
         """ Answer to a packet by re-using its ID for the packet answer """
+        assert packet.isResponse(), packet
         if self.isClosed():
-            raise ConnectionClosed
+            if packet.ignoreOnClosedConnection() and not packet.isError():
+                raise ConnectionClosed
+            return
         packet.setId(self.peer_id)
-        assert packet.isResponse(), packet
         self._addPacket(packet)
 
     def idle(self):