class WWW::Mechanize::Chain::CustomHeaders

def handle(ctx, params)
  request = params[:request]
  params[:headers].each do |k,v|
    case k
    when :etag then request.add_field("ETag", v)
    when :if_modified_since then request.add_field("If-Modified-Since", v)
    else
      raise ArgumentError.new("unknown header symbol #{k}") if k.is_a? Symbol
      request.add_field(k,v)
    end
  end
  super
end