Manipulation: use .children to select tbody elements

- selectors beginning with a child combinator are not valid natively.
  This fixes the tests when using selector-native.js

src/manipulation.js

@@ -52,7 +52,7 @@ function manipulationTarget( elem, content ) {
 	if ( nodeName( elem, "table" ) &&
 		nodeName( content.nodeType !== 11 ? content : content.firstChild, "tr" ) ) {
 
-		return jQuery( ">tbody", elem )[ 0 ] || elem;
+		return jQuery( elem ).children( "tbody" )[ 0 ] || elem;
 	}
 
 	return elem;