Made isObjectLiteral to work correctly with custom objects with empty prototypes.

src/core.js

@@ -328,13 +328,20 @@ jQuery.extend({
 			return false;
 		}
 		
+		// 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
-		
+	
 		var key;
 		for ( key in obj ) {}
 		
-		return !key || hasOwnProperty.call( obj, key );
+		return key === undefined || hasOwnProperty.call( obj, key );
 	},
 
 	isEmptyObject: function( obj ) {

test/unit/core.js

@@ -553,10 +553,14 @@ test("jQuery.extend(Object, Object)", function() {
 	same( empty.foo, optionsWithDate.foo, "Dates copy correctly" );
 
 	var myKlass = function() {};
+	var optionsWithCustomObject = { foo: { date: new myKlass } };
+	empty = {};
+	jQuery.extend(true, empty, optionsWithCustomObject);
+	same( empty.foo, optionsWithCustomObject.foo, "Custom objects copy correctly (no methods)" );
+	
 	// Makes the class a little more realistic
 	myKlass.prototype = { someMethod: function(){} };
 	empty = {};
-	var optionsWithCustomObject = { foo: { date: new myKlass } };
 	jQuery.extend(true, empty, optionsWithCustomObject);
 	same( empty.foo, optionsWithCustomObject.foo, "Custom objects copy correctly" );