Tests: fixed mail_imap_ssl.t too long shutdown. Prior to literals support in IMAP test backend (e7f0b4ca0a1a), early backend response was treated as invalid, with subsequent proxy connection close. Now that the connection continues successfully, this requires connection close before nginx shutdown. Otherwise, it would wait for proxy_timeout.

--- a/mail_imap_ssl.t
+++ b/mail_imap_ssl.t
@@ -207,7 +207,7 @@ my $s = Test::Nginx::IMAP->new(PeerAddr 
 );
 $s->ok('trusted cert');
 $s->send('1 AUTHENTICATE PLAIN ' . $cred->("s5"));
-$s->read();
+undef $s;
 
 # test auth_http request header fields with access_log