Fix Hackney adapter

Because header keys are case-insensitive in both HTTP/1.1 and HTTP/2, it is recommended for header keys to be in lowercase, to avoid sending duplicate keys in a request.

lib/twirp/client/hackney.ex

@@ -28,7 +28,7 @@ defmodule Twirp.Client.Hackney do
 
     with {:ok, status, headers, ref} <- :hackney.request(:post, path, ctx.headers, payload, options),
          {:ok, body} <- :hackney.body(ref) do
-      {:ok, %{status: status, headers: headers, body: body}}
+      {:ok, %{status: status, headers: format_headers(headers), body: body}}
     else
       {:error, :timeout} ->
         {:error, %{reason: :timeout}}
@@ -43,4 +43,10 @@ defmodule Twirp.Client.Hackney do
         error
     end
   end
+
+  defp format_headers(headers) do
+    for {key, value} <- headers do
+      {String.downcase(to_string(key)), to_string(value)}
+    end
+  end
 end

lib/twirp/plug.ex

@@ -81,8 +81,7 @@ defmodule Twirp.Plug do
 
   def call(%{path_info: [full_name, method]}=conn, {%{full_name: full_name}=service, handler, hooks}) do
     env = %{}
-    metadata = %{
-    }
+    metadata = %{}
     start = Telemetry.start(:call, metadata)
 
     try do

test/twirp/client/hackney_test.exs

@@ -87,6 +87,23 @@ defmodule Twirp.Client.HackneyTest do
     assert match?(%Error{code: :internal}, resp)
   end
 
+  test "normalize headers", %{service: service} do
+    Bypass.expect(service, fn conn ->
+      assert Plug.Conn.get_req_header(conn, "content-type") == ["application/protobuf"]
+      {:ok, body, conn} = Plug.Conn.read_body(conn)
+      assert %Req{msg: "test"} == Req.decode(body)
+
+      body = Resp.encode(Resp.new(msg: "test"))
+
+      conn
+      |> Plug.Conn.put_resp_header("Content-Type", "application/protobuf")
+      |> Plug.Conn.resp(200, body)
+    end)
+
+    resp = Client.echo(Req.new(msg: "test"))
+    assert {:ok, Resp.new(msg: "test")} == resp
+  end
+
   test "no headers are returned", %{service: service} do
     Bypass.expect(service, fn conn ->
       conn