Method append

src/test-utils.ts:218–222

(key: string, val: string)

    return super.set(key.toLowerCase(), value)
  }
  append(key: string, val: string) {
    let prev = this.get(key) || ''
    if (prev) prev += ', '
    this.set(key, prev + val)
  }
}

// @ts-ignore - faking it