The %s handler read LiquidDate.getTime(), which returns the
displayDate deliberately shifted by the display timezone offset for
wall-clock getters. With a timezone argument or timezoneOffset
option set, %s produced an epoch shifted by (server offset - display
offset) instead of the true Unix timestamp.
Expose the unshifted time as LiquidDate.dateValue() and use it for
%s. Also switch Math.round to Math.floor so fractional seconds
truncate toward the epoch like Ruby strftime.
Fixes#931
