Test four README settings pixa does not have yet (closes #61)

Failing tests for access_control_allow_origin, upstream_fetch_timeout,
upstream_max_response_size and downstream_timeout: their defaults,
valid values from the file and the environment, invalid values aborting
startup naming the key or variable and the value, the CORS middleware
answering with the configured origin, and the server's write timeout
coming from downstream_timeout. They do not compile until the settings
exist.

Model: opus-5-5

internal/server/http_internal_test.go

@@ -11,11 +11,15 @@ import (
 // carries every hardening timeout wired onto it, including the slowloris
 // defense (ReadHeaderTimeout) and the keep-alive bound (IdleTimeout). This
 // guards against a field being defined but never set on the server, so
-// each assertion compares the server field to its constant.
+// each assertion compares the server field to its constant, or, for
+// WriteTimeout, to downstream_timeout from the config.
 func TestNewHTTPServerTimeouts(t *testing.T) {
 	t.Parallel()
 
-	s := &Server{config: &config.Config{Port: 8080}}
+	s := &Server{config: &config.Config{
+		Port:              8080,
+		DownstreamTimeout: 45 * time.Second,
+	}}
 
 	srv := s.newHTTPServer()
 
@@ -26,7 +30,7 @@ func TestNewHTTPServerTimeouts(t *testing.T) {
 	}{
 		{"ReadTimeout", srv.ReadTimeout, HTTPReadTimeout},
 		{"ReadHeaderTimeout", srv.ReadHeaderTimeout, HTTPReadHeaderTimeout},
-		{"WriteTimeout", srv.WriteTimeout, HTTPWriteTimeout},
+		{"WriteTimeout", srv.WriteTimeout, 45 * time.Second},
 		{"IdleTimeout", srv.IdleTimeout, HTTPIdleTimeout},
 	}