add RRruleSet.toJSON method

Author: leigh-johnson

lib/rrule.js

@@ -1835,6 +1835,13 @@
       return JSON.stringify(this.valueOf())
     },
 
+    /**
+    * invoked when instance is passed through JSON.stringify()
+    */
+    toJSON: function () {
+      return JSON.stringify(this.valueOf())
+    },
+
     _iter: function (iterResult) {
       var _exdateHash = {}
       var _exrule = this._exrule