Make request header normalization configurable

[yanomateus]

trafficstars

Hi there!

Request headers are always downcased when sent upstream here.

  defp normalize_headers(headers) do
    headers
    |> downcase_headers
    |> remove_hop_by_hop_headers
  end

Could the casing be configurable? The upstream we relay requests to through this plug performs case sensitive matching to consume some headers, dropping the ones which have been downcased.

Thanks!