append (field, val) {
    const prev = this.get(field)

    if (prev) {
      val = Array.isArray(prev)
        ? prev.concat(val)
        : [prev].concat(val)
    }

    return this.set(field, val)
  }
Append additional header `field` with value `val`.

Examples:

```
this.append('Link', ['<http://localhost/>', '<http://localhost:3000/>']);
this.append('Set-Cookie', 'foo=bar; Path=/; HttpOnly');
this.append('Warning', '199 Miscellaneous warning');
```

@param {String} field
@param {*} val
@api public