forms: Better failsafe for __toString

This mostly helps when fields change types. If the type of the field
currently cannot convert previous data to a string (like if a short-answer
field was changed to choices), the textual representation of the PHP value
of the field data is given.
Merge pull request #659 from greezybacon/issue/652

forms: Better failsafe for __toString

Reviewed-By: Peter Rotich <peter@osticket.com>