Escape late rather than during building of query_string

[phh]

To reproduce:

• Go to pages
• In the admin columns click an author (to show the authors posts)
• Then click "Sort by order"
• The filter is now broken as the url instead contains #038;
• Replacing #038; with & fixes the filter

With this fix the filters just keep working.

[helen]

Hi @phh! Sorry we missed this - we had been working on migrating this plugin repo to the @10up organization account. Looks like an identical fix went in as 10up/simple-page-ordering#6 and was released to .org - I don't know if there's any way to get you GitHub contribution credit at this point without the unpleasantries of force-pushing but I'll look into it.