added json support to Rx

resolves the value (for native types) or "toJson()" in custom classes.

 import 'dart:async';
 import 'dart:collection';
+
 import '../rx_core/rx_interface.dart';
 
 RxInterface getObs;
@@ -102,6 +103,8 @@ class _RxImpl<T> implements RxInterface<T> {
   @override
   String toString() => value.toString();
 
+  dynamic toJson() => value;
+
   /// This equality override works for _RxImpl instances and the internal values.
   @override
   bool operator ==(dynamic o) {
@@ -285,6 +288,9 @@ class Rx<T> extends _RxImpl<T> {
   Rx([T initial]) {
     _value = initial;
   }
+
+  @override
+  dynamic toJson() => (value as dynamic)?.toJson();
 }
 
 extension StringExtension on String {