LP#1311467: fix heading validation in bib editor

This patch fixes a regression whereby clicking the validate button
in the bib editor would result in no headings getting checked.

In the process, it enables 'startwith' to be used as a JSON query
operator for all types of predicates, not just the results of
transformed values.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
Signed-off-by: Mike Rylander <mrylander@gmail.com>