core: Fixed #2605: .data() now accepts null as a value.

src/core.js

@@ -669,7 +669,7 @@ jQuery.extend({
 			jQuery.cache[ id ] = {};
 		
 		// Prevent overriding the named cache with undefined values
-		if ( data != undefined )
+		if ( data !== undefined )
 			jQuery.cache[ id ][ name ] = data;
 		
 		// Return the named cache data, or the ID for the element	

test/unit/core.js

@@ -1398,13 +1398,17 @@ test("$.className", function() {
 });
 
 test("$.data", function() {
-	expect(3);
+	expect(5);
 	var div = $("#foo")[0];
 	ok( jQuery.data(div, "test") == undefined, "Check for no data exists" );
 	jQuery.data(div, "test", "success");
 	ok( jQuery.data(div, "test") == "success", "Check for added data" );
 	jQuery.data(div, "test", "overwritten");
 	ok( jQuery.data(div, "test") == "overwritten", "Check for overwritten data" );
+	jQuery.data(div, "test", undefined);
+	ok( jQuery.data(div, "test") == "overwritten", "Check that data wasn't removed");
+	jQuery.data(div, "test", null);
+	ok( jQuery.data(div, "test") === null, "Check for null data");
 });
 
 test(".data()", function() {