Fix for servers with LOGINDISABLED before STARTTLS

When connecting to a server that advertises LOGINDISABLED in its
CAPABILITIES before STARTTLS, the connection raises an error and
destroys the socket before completing STARTTLS.  This is demonstrated in
the following debug output:

debug: [connection] Connected to host
debug: <= '* OK [CAPABILITY IMAP4rev1 LITERAL+ SASL-IR LOGIN-REFERRALS ID ENABLE IDLE STARTTLS LOGINDISABLED] Dovecot ready.'
debug: => 'A0 CAPABILITY'
debug: <= '* CAPABILITY IMAP4rev1 LITERAL+ SASL-IR LOGIN-REFERRALS ID ENABLE IDLE STARTTLS LOGINDISABLED'
debug: <= 'A0 OK Pre-login capabilities listed, post-login capabilities have more.'
error: Error: Logging in is disabled on this server source=authentication
debug: [connection] Closed

It appears that a return after _starttls was overlooked (since _starttls
will call _login again once the STARTTLS has completed).

Signed-off-by: Kevin Locke <klocke@quantpost.com>

lib/Connection.js

@@ -1463,8 +1463,10 @@ Connection.prototype._login = function() {
       if (self.serverSupports('STARTTLS')
           && (self._config.autotls === 'always'
               || (self._config.autotls === 'required'
-                  && self.serverSupports('LOGINDISABLED'))))
+                  && self.serverSupports('LOGINDISABLED')))) {
           self._starttls();
+          return;
+      }
 
       if (self.serverSupports('LOGINDISABLED')) {
         err = new Error('Logging in is disabled on this server');