Fix BETWEEN filter not working

Bug report:
http://www.fabrikar.com/forums/index.php?threads/alphabetical-sorting-help.37012/

Note: Considered whether it was element->filter_type which was not being
set correctly, but actually cannot find many places where
$element->filter_type is actually set.

Also fixed a type $filter should be $filters.
Merge pull request #1126 from Sophist-UK/fix_url_filter_between

Fix BETWEEN filter not working