code: function stringToSign (options) {
    var headers = options.amazonHeaders || ''
    if (headers) headers += '\n'
    var r = 
      [ options.verb
      , options.md5
      , options.contentType
      , options.date ? options.date.toUTCString() : ''
      , headers + options.resource
      ]
    return r.join('\n')
  }
docs: Return a string for sign() with the given `options`.
  
  Spec:
  
  <verb>\n
  <md5>\n
  <content-type>\n
  <date>\n
  [headers\n]
  <resource>
  
  @param {Object} options
  @return {String}
  @api private