Issue #4471: Properly shutdown socket in IMAP.shutdown().  Patch by
Lorenzo M. Catucci.
diff --git a/Lib/imaplib.py b/Lib/imaplib.py
index 1b37546..e020747 100644
--- a/Lib/imaplib.py
+++ b/Lib/imaplib.py
@@ -260,6 +260,7 @@
     def shutdown(self):
         """Close I/O established in "open"."""
         self.file.close()
+        self.sock.shutdown(socket.SHUT_RDWR)
         self.sock.close()
 
 
diff --git a/Lib/test/test_imaplib.py b/Lib/test/test_imaplib.py
index 8d86ce6..351659d 100644
--- a/Lib/test/test_imaplib.py
+++ b/Lib/test/test_imaplib.py
@@ -217,6 +217,7 @@
 
     def test_logout(self):
         rs = self.server.logout()
+        self.server = None
         self.assertEqual(rs[0], 'BYE')