Core: restore enumeration behavior in isPlainObject

Fixes jquerygh-2968
Close jquerygh-2970

Author: timmywil

src/core.js

@@ -223,6 +223,7 @@ jQuery.extend( {
 	},
 
 	isPlainObject: function( obj ) {
+		var key;
 
 		// Not plain objects:
 		// - Any object or value whose internal [[Class]] property is not "[object Object]"
@@ -237,9 +238,11 @@ jQuery.extend( {
 			return false;
 		}
 
-		// If the function hasn't returned already, we're confident that
-		// |obj| is a plain object, created by {} or constructed with new Object
-		return true;
+		// Own properties are enumerated firstly, so to speed up,
+		// if last one is own, then all properties are own
+		for ( key in obj ) {}
+
+		return key === undefined || hasOwn.call( obj, key );
 	},
 
 	isEmptyObject: function( obj ) {

test/unit/core.js

@@ -268,13 +268,21 @@ QUnit.test( "type for `Symbol`", function( assert ) {
 } );
 
 QUnit.asyncTest( "isPlainObject", function( assert ) {
-	assert.expect( 15 );
+	assert.expect( 19 );
 
-	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" );
@@ -302,6 +310,10 @@ QUnit.asyncTest( "isPlainObject", function( assert ) {
 	// Again, instantiated objects shouldn't be matched
 	assert.ok( !jQuery.isPlainObject( new fn() ), "new fn" );
 
+	// 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" );