Added ElementProxy::getOrAddElement() (fixes #1311)

extras/tests/ElementProxy/CMakeLists.txt

@@ -9,6 +9,7 @@ add_executable(ElementProxyTests
 	remove.cpp
 	set.cpp
 	size.cpp
+	subscript.cpp
 )
 
 add_test(ElementProxy ElementProxyTests)

extras/tests/ElementProxy/subscript.cpp

@@ -7,7 +7,7 @@
 
 using namespace ARDUINOJSON_NAMESPACE;
 
-TEST_CASE("MemberProxy::operator[]") {
+TEST_CASE("ElementProxy::operator[]") {
   DynamicJsonDocument doc(4096);
   ElementProxy<JsonDocument&> ep = doc[1];