examples: fix pbkdf2 invocation

Calling `crypto.pbkdf2()` without a digest has been deprecated in Node
and is scheduled to be broken in Node 8.

Fix this by actually passing a digest.

ref: nodejs/node#11305

examples/auth/pass.js

@@ -31,15 +31,15 @@ var iterations = 12000;
 
 exports.hash = function (pwd, salt, fn) {
   if (3 == arguments.length) {
-    crypto.pbkdf2(pwd, salt, iterations, len, function(err, hash){
+    crypto.pbkdf2(pwd, salt, iterations, len, 'sha256', function(err, hash){
       fn(err, hash.toString('base64'));
     });
   } else {
     fn = salt;
     crypto.randomBytes(len, function(err, salt){
       if (err) return fn(err);
       salt = salt.toString('base64');
-      crypto.pbkdf2(pwd, salt, iterations, len, function(err, hash){
+      crypto.pbkdf2(pwd, salt, iterations, len, 'sha256', function(err, hash){
         if (err) return fn(err);
         fn(null, salt, hash.toString('base64'));
       });