lib/http: fix tests after go1.23 update

go1.22 output the Content-Length on a bad Range request on a file but
go1.23 doesn't - adapt the tests accordingly.

lib/http/serve/serve_test.go

@@ -76,7 +76,9 @@ func TestObjectBadRange(t *testing.T) {
 	Object(w, r, o)
 	resp := w.Result()
 	assert.Equal(t, http.StatusBadRequest, resp.StatusCode)
-	assert.Equal(t, "10", resp.Header.Get("Content-Length"))
+	if contentLength := resp.Header.Get("Content-Length"); contentLength != "" {
+		assert.Equal(t, "10", contentLength)
+	}
 	body, _ := io.ReadAll(resp.Body)
 	assert.Equal(t, "Bad Request\n", string(body))
 }