To match formulas, 'formula is' condition should be used instead

Change-Id: I37a9a2680c49bffcb8b25a11faedc1421cfa2d00
Reviewed-on: https://gerrit.libreoffice.org/18126Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Tested-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Reviewed-on: https://gerrit.libreoffice.org/18159Reviewed-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>