LP#1292705 Fix broken is*() methods in fm_IDL2js.xsl

A thinko meant that we were no longer incrementing the field variable,
leading to breakage in interfaces that used isnew() and the like.

Signed-off-by: Dan Scott <dscott@laurentian.ca>
Signed-off-by: Ben Shum <bshum@biblio.org>