fix #1097 better

If there is

* no existing filters on the request, OR
* filters, but none of them are a sort

then extend the filters with a sort=-modified.

It turns out that the filters datastructure is more inspectable
than I had originally recalled, presumably predicting something just
like (actually this: http://trac.tiddlywiki.org/ticket/1161 ).

@colmjude, @pads, @fnd this good?