Attributes: return null when attribute does not exist

Fixes gh-2118
Close gh-2129

Conflicts:
	test/unit/attributes.js

src/attributes/attr.js

@@ -66,12 +66,7 @@ jQuery.extend({
 			return ret;
 
 		} else {
-			ret = jQuery.find.attr( elem, name );
-
-			// Non-existent attributes return null, we normalize to undefined
-			return ret == null ?
-				undefined :
-				ret;
+			return jQuery.find.attr( elem, name );
 		}
 	},