Do not check for 'AUTH=PLAIN' capability when attempting to do plain-text authentication.

As per RFC 3501, all IMAP servers must implement at least plain-text authentication, whether they advertise the capability or not.

imap.js

@@ -960,9 +960,8 @@ ImapConnection.prototype._login = function(cb) {
 
     if (this.capabilities.indexOf('AUTH=XOAUTH') !== -1 && 'xoauth' in this._options)
       this._send('AUTHENTICATE XOAUTH ' + escape(this._options.xoauth), fnReturn);
-    else if (this.capabilities.indexOf('AUTH=PLAIN') !== -1 ||
-             // Kludge due to GMail no longer advertising plaintext auth ...
-             this.capabilities.indexOf('X-GM-EXT-1') !== -1) {
+    else if (this._options.username !== undefined &&
+             this._options.password !== undefined) {
       this._send('LOGIN "' + escape(this._options.username) + '" "'
                  + escape(this._options.password) + '"', fnReturn);
     } else {