code: function validateHeader (key, value) {
    if (typeof key !== 'string' || typeof value !== 'string')
      return option.some(`Parts of header ${key}:${value} must be strings`)
    return option.none
  }
docs: Validate header to all parts be strings
  @param {String} key – Header key
  @param {String} value – Header value
  @return {String} error