Exclude undefined keys

Today params creates undefined attributes for each whitelisted value.

```js
actual = params({ foo: 'bar', baz: 'zo', cl: 'fn' }).only('foo', 'baz', 'not_defined');
{ foo: 'bar', baz: 'zo', not_defined: undefined }
```

This breaks our filtering as not send values are now present as undefined.
I'm guilty of doing this, but I have come around to the fact that it is a terrible way to do this kind of check. I would suggest either `.hasOwnProperty` or `key in hash` for more consistent results.