Phoenix: capitalize header keys

capitalize_header_keys.ex

defmodule CapitalizeHeaderKeys do
  def hook(status, headers, body, req) do
    headers = capitalize_resp_headers(headers)
    {:ok, req} = :cowboy_req.reply(status, headers, body, req)
    req
  end

  # Cowboy v1
  defp capitalize_resp_headers(headers) when is_list(headers) do
    :lists.keymap(&:cowboy_bstr.capitalize_token/1, 1, headers)
  end

  # Cowboy v2
  defp capitalize_resp_headers(headers) when is_map(headers) do
    Enum.map(headers, fn {k, v} -> {:cowboy_bstr.capitalize_token(k), v} end)
  end
end

config.exs

use Mix.Config

config :my_app, MyApp.Endpoint,
  debug_errors: false,
  http: [
    protocol_options: [
      onresponse: {CapitalizeHeaderKeys, :hook}
    ]
  ]