Ajax: Use two-arg form of .slice() for old IE

Followup to a4b9bc57, #14773

src/ajax/load.js

@@ -25,7 +25,7 @@ jQuery.fn.load = function( url, params, callback ) {
 		off = url.indexOf(" ");
 
 	if ( off >= 0 ) {
-		selector = jQuery.trim( url.slice( off ) );
+		selector = jQuery.trim( url.slice( off, url.length ) );
 		url = url.slice( 0, off );
 	}