Dropdown: Fix title not updated. (#851)

A bug was introduced by:
https://github.com/ArthurSonzogni/FTXUI/pull/826

The checkbox label wasn't updated.

Bug:https://github.com/ArthurSonzogni/FTXUI/issues/861

src/ftxui/util/ref_test.cpp

@@ -0,0 +1,63 @@
+#include "ftxui/util/ref.hpp"
+
+#include <gtest/gtest.h>
+#include "ftxui/component/component.hpp"
+
+namespace ftxui {
+namespace {
+class Adapter : public ConstStringListRef::Adapter {
+ public:
+  Adapter(std::vector<std::string>& entries) : entries(entries) {}
+  size_t size() const override { return entries.size() * 2; }
+  std::string operator[](size_t index) const override {
+    return entries[index / 2];
+  }
+  std::vector<std::string>& entries;
+};
+}  // namespace
+
+TEST(ConstStringListRef, Copy) {
+  std::vector<std::string> entries = {
+      "entry 1",
+      "entry 2",
+      "entry 3",
+  };
+  int selected = 0;
+  auto menu = Menu(entries, &selected);
+}
+
+TEST(ConstStringListRef, Ref) {
+  std::vector<std::string> entries = {
+      "entry 1",
+      "entry 2",
+      "entry 3",
+  };
+  int selected = 0;
+  auto menu = Menu(&entries, &selected);
+}
+
+TEST(ConstStringListRef, Adapter) {
+  std::vector<std::string> entries = {
+      "entry 1",
+      "entry 2",
+      "entry 3",
+  };
+
+  int selected = 0;
+  Adapter a(entries);
+  auto menu = Menu(&a, &selected);
+}
+
+TEST(ConstStringListRef, UniquePtrAdapter) {
+  std::vector<std::string> entries = {
+      "entry 1",
+      "entry 2",
+      "entry 3",
+  };
+
+  int selected = 0;
+  auto a = std::make_unique<Adapter>(entries);
+  auto menu = Menu(std::move(a), &selected);
+}
+
+}  // namespace ftxui