Adds the possibility to assert cookie attributes with the following
syntax:

```
expect(res).to.have.cookie('key', 'value', {
  'AttrOne': 'value-one',
  'AttrTwo': 'value-two',
  // ...
});
```

When using .cookie(key, val, attributes), the assertion context will
change from the original object to the cookie itself, in order to allow
readable chained assertions about the cookie properties.