add a test case for JSON.stringify() with wrapped class (issue #88)

According to the [`JSON.stringify()` documentation][1] it's possible to add a
function `toJSON()` to prototype for custom conversion to JSON.

Added a `Xbase::to_json()` functions and exposed it as `toJSON()` into v8,
so that `JSON.stringify()` call for such objects would invoke the conversion
function.

Updated `create_X()` factory function to set the `X::var` on construction at
JavaScript side.

[1]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/JSON/stringify#toJSON()_behavior

test/test_class.cpp

@@ -8,6 +8,7 @@
 //
 #include "v8pp/class.hpp"
 #include "v8pp/module.hpp"
+#include "v8pp/object.hpp"
 #include "v8pp/property.hpp"
 
 #include "test.hpp"
@@ -27,16 +28,30 @@ struct Xbase
 	int fun3(int x) volatile { return var + x; }
 	int fun4(int x) const volatile { return var + x; }
 	static int static_fun(int x) { return x; }
+
+	v8::Local<v8::Value> to_json(v8::Isolate* isolate, v8::Local<v8::Value> key) const
+	{
+		v8::EscapableHandleScope scope(isolate);
+		v8::Local<v8::Object> result = v8::Object::New(isolate);
+		v8pp::set_const(isolate, result, "key", key);
+		v8pp::set_const(isolate, result, "var", var);
+		return scope.Escape(result);
+	}
 };
 
 struct X : Xbase
 {
 };
 
 template<typename Traits, typename X_ptr = typename v8pp::class_<X, Traits>::object_pointer_type>
-static X_ptr create_X(v8::FunctionCallbackInfo<v8::Value> const&)
+static X_ptr create_X(v8::FunctionCallbackInfo<v8::Value> const& args)
 {
-	return X_ptr(new X);
+	X_ptr x(new X);
+	if (args.Length() > 0)
+	{
+		x->var = v8pp::from_v8<int>(args.GetIsolate(), args[0]);
+	}
+	return x;
 }
 
 struct Y : X
@@ -124,6 +139,7 @@ void test_class_()
 		.set("static_fun", &X::static_fun)
 		.set("static_lambda", [](int x) { return x + 3; })
 		.set("extern_fun", extern_fun<Traits>)
+		.set("toJSON", &X::to_json)
 		;
 
 	v8pp::class_<Y, Traits> Y_class(isolate);
@@ -165,6 +181,11 @@ void test_class_()
 	check_eq("X::extern_fun(5)", run_script<int>(context, "x = new X(); x.extern_fun(5)"), 6);
 	check_eq("X::extern_fun(6)", run_script<int>(context, "X.extern_fun(6)"), 6);
 
+	check_eq("JSON.stringify(X)",
+		run_script<std::string>(context, "JSON.stringify({'obj': new X(10), 'arr': [new X(11), new X(12)] })"),
+		R"({"obj":{"key":"obj","var":10},"arr":[{"key":"0","var":11},{"key":"1","var":12}]})"
+	);
+
 	check_eq("Y object", run_script<int>(context, "y = new Y(-100); y.konst + y.var"), -1);
 
 	auto y1 = v8pp::factory<Y, Traits>::create(isolate, -1);