fix(cursor): allow $meta based sort when passing an array to sort()

You may pass an object with a `$meta` key in place of the order parameter to `sort`.  The previous code would incorrectly reject this input as illegal.  See https://docs.mongodb.com/manual/reference/method/cursor.sort/#sort-metadata for documentation.

lib/cursor.js

@@ -557,7 +557,7 @@ Cursor.prototype.sort = function(keyOrList, direction) {
           value[1] = 1;
         } else if (x[1] === 'desc') {
           value[1] = -1;
-        } else if (x[1] === 1 || x[1] === -1) {
+        } else if (x[1] === 1 || x[1] === -1 || x[1].$meta) {
           value[1] = x[1];
         } else {
           throw new MongoError(