martian(header): do not add X-Forwarded headers for CONNECT requests

internal/martian/header/forwarded_modifier.go

@@ -33,6 +33,10 @@ import (
 func NewForwardedModifier() martian.RequestModifier {
 	return martian.RequestModifierFunc(
 		func(req *http.Request) error {
+			if req.Method == http.MethodConnect {
+				return nil
+			}
+
 			if v := req.Header.Get("X-Forwarded-Proto"); v == "" {
 				req.Header.Set("X-Forwarded-Proto", req.URL.Scheme)
 			}

internal/martian/header/forwarded_modifier_test.go

@@ -82,4 +82,23 @@ func TestSetForwardHeaders(t *testing.T) {
 	if got, want := req.Header.Get(xfu), "https://preserved.host.com/foo?x=y"; got != want {
 		t.Errorf("req.Header.Get(%q): got %q, want %q", xfh, got, want)
 	}
+
+	// Test that the modifier does not set headers for CONNECT requests.
+	req, err = http.NewRequest(http.MethodConnect, "http://martian.local", http.NoBody)
+	if err != nil {
+		t.Fatalf("http.NewRequest(): got %v, want no error", err)
+	}
+
+	if got, want := req.Header.Get(xfp), ""; got != want {
+		t.Errorf("req.Header.Get(%q): got %q, want %q", xfp, got, want)
+	}
+	if got, want := req.Header.Get(xff), ""; got != want {
+		t.Errorf("req.Header.Get(%q): got %q, want %q", xff, got, want)
+	}
+	if got, want := req.Header.Get(xfh), ""; got != want {
+		t.Errorf("req.Header.Get(%q): got %q, want %q", xfh, got, want)
+	}
+	if got, want := req.Header.Get(xfu), ""; got != want {
+		t.Errorf("req.Header.Get(%q): got %q, want %q", xfh, got, want)
+	}
 }