[fix] stubs restore prototype props correctly

closes #2226

Author: fatso83

lib/sinon/default-behaviors.js

@@ -258,7 +258,7 @@ var defaultBehaviors = {
         Object.defineProperty(rootStub.rootObj, rootStub.propName, {
             value: newVal,
             enumerable: true,
-            configurable: isPropertyConfigurable(rootStub.rootObj, rootStub.propName)
+            configurable: rootStub.shadowsPropOnPrototype || isPropertyConfigurable(rootStub.rootObj, rootStub.propName)
         });
 
         return fake;

lib/sinon/stub.js

@@ -94,11 +94,14 @@ function stub(object, property) {
     var func = typeof actualDescriptor.value === "function" ? actualDescriptor.value : null;
     var s = createStub(func);
 
+    var propIsOwn = Boolean(actualDescriptor.isOwn);
+
     extend.nonEnum(s, {
         rootObj: object,
         propName: property,
+        shadowsPropOnPrototype: !propIsOwn,
         restore: function restore() {
-            if (actualDescriptor !== undefined) {
+            if (actualDescriptor !== undefined && propIsOwn) {
                 Object.defineProperty(object, property, actualDescriptor);
                 return;
             }