headerSort = function(a, b) {
    // Headers are sorted lexigraphically based on the header name only.
    a = a.split(":")[0]
    b = b.split(":")[0]

    return a > b ? 1 : -1;
  }
Perform the following:

 - ignore non-amazon headers
 - lowercase fields
 - sort lexicographically
 - trim whitespace between ":"
 - join with newline

@param {Object} headers
@return {String}
@api private