# File lib/mechanize/http/agent.rb, line 487
  def request_add_headers request, headers = {}
    @request_headers.each do |k,v|
      request[k] = v
    end

    headers.each do |field, value|
      case field
      when :etag              then request["ETag"] = value
      when :if_modified_since then request["If-Modified-Since"] = value
      when Symbol then
        raise ArgumentError, "unknown header symbol #{field}"
      else
        request[field] = value
      end
    end
  end