Add 'walker' to defaults in wp_list_pages

[josh23french]

(r11999 2.9-rare)

There is a way that a theme developer can use his own walker in wp_list_pages, or there seems to be one. The only thing blocking this is that wp_parse_args only compacts the default keys and walker is not one of them. And I really don't feel like hacking core for this theme. It's only 1 line!

[scribu]

You're right: walk_page_tree() has a 'walker' argument which isn't propagated from wp_list_pages().

[ryan]

(In [12149]) Propagate walker arg down the stack. Props scribu. fixes #10921