allow change schema without touch current json_document

jsonform.gadget.js

@@ -661,6 +661,7 @@
           return json_document;
         })
         .push(function (json_d) {
+          gadget.state.value = JSON.stringify(json_d);
           return tv4.validateMultiple(json_d, gadget.props.schema[""]);
         })
         .push(function (validation) {
@@ -764,14 +765,17 @@
     })
 
     .declareMethod('render', function (options) {
-      return this.changeState({
+      var z = {
         key: options.key,
-        value: JSON.stringify(options.value),
         schema: JSON.stringify(options.schema),
         saveOrigValue: options.saveOrigValue,
         schema_url: options.schema_url,
         editable: options.editable === undefined ? true : options.editable
-      });
+      };
+      if (options.value !== undefined) {
+        z.value = JSON.stringify(options.value);
+      }
+      return this.changeState(z);
     })
     .onStateChange(function () {
       var g = this,