Core: restore enumeration behavior in isPlainObject

Fixes jquerygh-2968
Close jquerygh-2970

Author: timmywil

test/unit/core.js

@@ -328,13 +328,21 @@ QUnit.test( "type for `Symbol`", function( assert ) {
 } );
 
 QUnit.asyncTest( "isPlainObject", function( assert ) {
-	assert.expect( 16 );
+	assert.expect( 20 );
 
-	var pass, iframe, doc,
+	var pass, iframe, doc, parentObj, childObj, deep,
 		fn = function() {};
 
 	// The use case that we want to match
 	assert.ok( jQuery.isPlainObject( {} ), "{}" );
+	assert.ok( jQuery.isPlainObject( new window.Object() ), "new Object" );
+
+	parentObj = { foo: "bar" };
+	childObj = Object.create( parentObj );
+
+	assert.ok( !jQuery.isPlainObject( childObj ), "isPlainObject(Object.create({}))" );
+	childObj.bar = "foo";
+	assert.ok( !jQuery.isPlainObject( childObj ), "isPlainObject(Object.create({}))" );
 
 	// Not objects shouldn't be matched
 	assert.ok( !jQuery.isPlainObject( "" ), "string" );
@@ -372,6 +380,10 @@ QUnit.asyncTest( "isPlainObject", function( assert ) {
 
 	assert.ok( !jQuery.isPlainObject( new fn() ), "fn (inherited and own properties)" );
 
+	// Deep object
+	deep = { "foo": { "baz": true }, "foo2": document };
+	assert.ok( jQuery.isPlainObject( deep ), "Object with objects is still plain" );
+
 	// DOM Element
 	assert.ok( !jQuery.isPlainObject( document.createElement( "div" ) ), "DOM Element" );