code: function canonicalizeHeaders (headers) {
    var buf = []
      , fields = Object.keys(headers)
      ;
    for (var i = 0, len = fields.length; i < len; ++i) {
      var field = fields[i]
        , val = headers[field]
        , field = field.toLowerCase()
        ;
      if (0 !== field.indexOf('x-amz')) continue
      buf.push(field + ':' + val)
    }
    return buf.sort().join('\n')
  }
docs: Perform the following:
  
  - ignore non-amazon headers
  - lowercase fields
  - sort lexicographically
  - trim whitespace between ":"
  - join with newline
  
  @param {Object} headers
  @return {String}
  @api private