Pass params from doUntil/doWhilst callbacks to the test function

Pretty simple, it just passes the callback results from the `doWhilst` and `doUntil` iterators to the test function.  This allows you to do something like (contrived example):

```javascript
async.doWhilst(pollForSomething, _.isUndefined, callback);
```
rather than

```javascript
var value;
async.doWhilst(function(cb) {
  pollForSomething(function (err, res) {
    if(err) return cb(err);
    value = res;
    cb();
  });
},
function test() {
  return !value;
}, 
callback);
```