Add test that reproduces error without fix

tests/test_connection.py

@@ -732,6 +732,18 @@ async def handler(request):
             await client_ws.send_message('Hello from client!')
 
 
+async def test_server_sends_after_close(nursery):
+    async def handler(request):
+        server_ws = await request.accept()
+        with pytest.raises(ConnectionClosed):
+            await server_ws.send_message("Hello from server")
+    server = await nursery.start(serve_websocket, handler, HOST, 0, None)
+    stream = await trio.open_tcp_stream(HOST, server.port)
+    client_ws = await wrap_client_stream(nursery, stream, HOST, RESOURCE)
+    async with client_ws:
+        await stream.aclose()
+
+
 async def test_server_does_not_close_handshake(nursery):
     async def handler(stream):
         request = await wrap_server_stream(nursery, stream)