Required assert should only check if value is defined 

I think that the assert is doing more than it should. This shouldn't be invalid:

``` js
validator.validate({
  foobar: [],
  foobiz: ''
}, {
  foobar: [
    new Assert().Required(),
    new Assert().InstanceOf(Array)
  ],
  foobiz: [
    new Assert().Required()
  ]
});
```

@guillaumepotier WDYT? Currently I'm overriding the assert but if you agree I can create a PR.
