Fixed #15802 -- pyscopg2 sometimes fail to close the connection when it's...

Fixed #15802 -- pyscopg2 sometimes fail to close the connection when it's already closed by the server, Thanks Rick van Hattem

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16708 bcc190cf-cafb-0310-a4f2-bffc1f526a37