Refactor header validation in adapter_inform function

Author: josevalim

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!(to_string(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