code: function normalize (url, opts, callback) {
    url = typeof url === 'string' ? {url: url} : url
    opts = extend(url, opts)
  
    if (typeof opts.url !== 'string') {
      throw new TypeError('request-all: missing request url')
    }
    if (typeof callback !== 'function') {
      throw new TypeError('request-all: expect a callback')
    }
  
    opts.url = normalizeUrl(opts.url)
    opts.json = typeof opts.json === 'boolean' ? opts.json : true
    opts.headers = extend({
      'accept': 'application/json',
      'user-agent': 'https://github.com/tunnckoCore/request-all'
    }, opts.headers)
  
    return opts
  }
docs: > Normalize and validate arguments and options
  
  @param  {String|Object} `url`
  @param  {Object} `opts`
  @param  {Function} `callback`
  @return {Object}