Allow using a JsonVariant as a key or index

Closes #2080

extras/tests/JsonObject/containsKey.cpp

@@ -30,4 +30,10 @@ TEST_CASE("JsonObject::containsKey()") {
     REQUIRE(true == obj.containsKey(vla));
   }
 #endif
+
+  SECTION("key is a JsonVariant") {
+    doc["key"] = "hello";
+    REQUIRE(true == obj.containsKey(obj["key"]));
+    REQUIRE(false == obj.containsKey(obj["hello"]));
+  }
 }

extras/tests/JsonObject/remove.cpp

@@ -80,4 +80,10 @@ TEST_CASE("JsonObject::remove()") {
     JsonObject unboundObject;
     unboundObject.remove(unboundObject.begin());
   }
+
+  SECTION("remove(JsonVariant)") {
+    obj["key"] = "b";
+    obj.remove(obj["key"]);
+    REQUIRE("{\"a\":0,\"c\":2,\"key\":\"b\"}" == doc.as<std::string>());
+  }
 }

extras/tests/JsonObject/subscript.cpp

@@ -249,4 +249,12 @@ TEST_CASE("JsonObject::operator[]") {
     REQUIRE(true == obj["hello"]["world"].is<int>());
     REQUIRE(false == obj["hello"]["world"].is<bool>());
   }
+
+  SECTION("JsonVariant") {
+    obj["hello"] = "world";
+    doc["key"] = "hello";
+
+    REQUIRE(obj[obj["key"]] == "world");
+    REQUIRE(obj[obj["foo"]] == nullptr);
+  }
 }