fix(date): %s returns Unix epoch unaffected by display timezone (#932)

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

docs/source/filters/date.md

@@ -34,6 +34,7 @@ The `date` filter is used to convert a timestamp into the specified format.
             * minutes: `-360` means `'+06:00'` and `360` means `'-06:00'`
             * timeZone ID: `Asia/Colombo` or `America/New_York`
     * See [here](https://en.wikipedia.org/wiki/List_of_tz_database_time_zones) for TZ database values
+* `%s` (seconds since the Unix epoch) identifies an instant rather than a wall-clock time, so it's not affected by the display timezone.
 
 ### Examples
 ```liquid