Make the reverse sort prefix '-' instead of '!'

lib/imap.js

@@ -814,12 +814,11 @@ ImapConnection.prototype._sort = function(which, sorts, options, cb) {
                       + 'Expected string. Got: ' + typeof criteria);
 
     var modifier = '';
-    if (criterion.charAt(0) === '!') {
+    if (criterion[0] === '-') {
       modifier = 'REVERSE ';
       criterion = criterion.substring(1);
     }
-    criterion = criterion.toUpperCase();
-    switch (criterion) {
+    switch (criterion.toUpperCase()) {
       case 'ARRIVAL':
       case 'CC':
       case 'DATE':