Allow atom header keys in inform/3

lib/plug/conn.ex

@@ -1436,9 +1436,12 @@ defmodule Plug.Conn do
   end
 
   defp adapter_inform(%Conn{adapter: {adapter, payload}}, status, headers) do
-    for {key, value} <- headers do
-      validate_header_key_value!(key, value)
-    end
+    headers =
+      for {key, value} <- headers do
+        key = to_string(key)
+        validate_header_key_value!(key, value)
+        {key, value}
+      end
 
     adapter.inform(payload, status, headers)
   end

test/plug/conn_test.exs

@@ -468,6 +468,11 @@
     end
   end
 
+  test "inform/3 accepts atom header keys (e.g. for websocket upgrades)" do
+    conn = conn(:get, "/foo") |> inform(101, [{:upgrade, "websocket"}, {:connection, "Upgrade"}])
+    assert {101, [{:upgrade, "websocket"}, {:connection, "Upgrade"}]} in sent_informs(conn)
+  end
+
   test "inform!/3 performs an informational request" do
     conn = conn(:get, "/foo") |> inform!(103, [{"link", "</style.css>; rel=preload; as=style"}])
     assert {103, [{"link", "</style.css>; rel=preload; as=style"}]} in sent_informs(conn)