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')
}
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