src: use MakeCallback() -> Call() in AsyncWorker

Change `AsyncWorker::OnOK()` and `AsyncWorker::OnError()` callbacks to
**NOT** use `MakeCallback()`. An ordinary function call
(`_callback::Call()`) is now correct.

PR-URL: #361
Refs: https://nodejs.org/api/n-api.html#n_api_napi_make_callback
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Gabriel Schulhof <gabriel.schulhof@intel.com>
Reviewed-By: Nicola Del Gobbo <nicoladelgobbo@NickNaso.local>

napi-inl.h

@@ -3588,11 +3588,11 @@ inline FunctionReference& AsyncWorker::Callback() {
 }
 
 inline void AsyncWorker::OnOK() {
-  _callback.MakeCallback(_receiver.Value(), std::initializer_list<napi_value>{});
+  _callback.Call(_receiver.Value(), std::initializer_list<napi_value>{});
 }
 
 inline void AsyncWorker::OnError(const Error& e) {
-  _callback.MakeCallback(_receiver.Value(), std::initializer_list<napi_value>{ e.Value() });
+  _callback.Call(_receiver.Value(), std::initializer_list<napi_value>{ e.Value() });
 }
 
 inline void AsyncWorker::SetError(const std::string& error) {