SF bug #471720: ThreadingMixIn/TCPServer forgets close

Solved with a helper method that calls finish_request() and then
close_request().  The code is by Max Neunhöffer.
diff --git a/Misc/NEWS b/Misc/NEWS
index e2e9683..e89b789 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -86,6 +86,9 @@
   which indicates whether output is intended for the header 'Q'
   encoding.
 
+- The SocketServer.ThreadingMixIn class now closes the request after
+  finish_request() returns.  (Not when it errors out though.)
+
 Tools/Demos
 
 - Demo/dns was removed.  It no longer serves any purpose; a package