Fixed incorrect handling of arguments in "auto" in case of empty callback

Author: Ilya Shatokhin

lib/async.js

@@ -510,7 +510,7 @@
     };
 
     async.auto = function (tasks, concurrency, callback) {
-        if (!callback) {
+        if (typeof arguments[1] === 'function') {
             // concurrency is optional, shift the args.
             callback = concurrency;
             concurrency = null;