Made isPlainObject() supporting null, undefined, and window values on IE too....

Made isPlainObject() supporting null, undefined, and window values on IE too. Also added some related tests. Fixes #5669.

src/core.js

@@ -425,19 +425,21 @@ jQuery.extend({
 	},
 
 	isPlainObject: function( obj ) {
-		if ( toString.call(obj) !== "[object Object]" || typeof obj.nodeType === "number" ) {
+		// Must be an Object.
+		// Because of IE, we also have to check the presence of the constructor property.
+		if ( !obj || toString.call(obj) !== "[object Object]" || !("constructor" in obj) ) {
 			return false;
 		}
 		
-		// not own constructor property must be Object
+		// Not own constructor property must be Object
 		if ( obj.constructor
 			&& !hasOwnProperty.call(obj, "constructor")
 			&& !hasOwnProperty.call(obj.constructor.prototype, "isPrototypeOf") ) {
 			return false;
 		}
 		
-		//own properties are iterated firstly,
-		//so to speed up, we can test last one if it is own or not
+		// Own properties are enumerated firstly, so to speed up,
+		// if last one is own, then all properties are own.
 	
 		var key;
 		for ( key in obj ) {}

test/unit/core.js

@@ -204,12 +204,22 @@ test("trim", function() {
 });
 
 test("isPlainObject", function() {
-	expect(7);
+	expect(14);
 
 	stop();
 
 	// The use case that we want to match
 	ok(jQuery.isPlainObject({}), "{}");
+	
+	// Not objects shouldn't be matched
+	ok(!jQuery.isPlainObject(""), "string");
+	ok(!jQuery.isPlainObject(0) && !jQuery.isPlainObject(1), "number");
+	ok(!jQuery.isPlainObject(true) && !jQuery.isPlainObject(false), "boolean");
+	ok(!jQuery.isPlainObject(null), "null");
+	ok(!jQuery.isPlainObject(undefined), "undefined");
+	
+	// Arrays shouldn't be matched
+	ok(!jQuery.isPlainObject([]), "array");
  
 	// Instantiated objects shouldn't be matched
 	ok(!jQuery.isPlainObject(new Date), "new Date");
@@ -231,6 +241,9 @@ test("isPlainObject", function() {
 
 	// DOM Element
 	ok(!jQuery.isPlainObject(document.createElement("div")), "DOM Element");
+	
+	// Window
+	ok(!jQuery.isPlainObject(window), "window");
  
 	var iframe = document.createElement("iframe");
 	document.body.appendChild(iframe);