Add implicit conversion from JsonDocument to JsonVariant

extras/tests/JsonDocument/cast.cpp

@@ -0,0 +1,18 @@
+// ArduinoJson - https://arduinojson.org
+// Copyright © 2014-2022, Benoit BLANCHON
+// MIT License
+
+#include <ArduinoJson.h>
+#include <catch.hpp>
+
+#include <string>
+
+TEST_CASE("Implicit cast to JsonVariant") {
+  StaticJsonDocument<128> doc;
+
+  doc["hello"] = "world";
+
+  JsonVariant var = doc;
+
+  CHECK(var.as<std::string>() == "{\"hello\":\"world\"}");
+}