Fixes issue #8065: Answer code conversion to '0'

DEV: when exporting to CSV/EXCEL etc. and conversion
DEV: of Y or N to '0' was chosen, values were wrongly
DEV: exported as 'missing values' (empty)
DEV:
DEV: nasty php typing problem....probably same problem
DEV: with issue #8039

Conflicts:
	application/helpers/common_helper.php
small change to also honor 'false'

thanks to Shnoulle
Merge pull request #101 from mfaber/zerovalue_master

Fixed issue #8065: Answer code conversion to '0'