fs: make fs.symlink() with no callback async

Fix a bug where fs.symlink('foo', 'bar') executed symlink(2) synchronously.

lib/fs.js

@@ -478,7 +478,7 @@ fs.readlinkSync = function(path) {
 fs.symlink = function(destination, path, type_, callback) {
   var type = (typeof(type_) == 'string' ? type_ : null);
   var callback_ = arguments[arguments.length - 1];
-  callback = (typeof(callback_) == 'function' ? callback_ : null);
+  callback = (typeof(callback_) == 'function' ? callback_ : noop);
 
   if (isWindows && type === 'junction') {
     destination = pathModule._makeLong(destination);