DSNBounce exception when no Date header

When the DSNBounce mailet processes a mail without a Date header, it fails with the exception

java.time.temporal.UnsupportedTemporalTypeException: Unsupported field: OffsetSeconds

because the LocalDateTime used to construct the new date does not include timezone information required by the date formatter.

The fix is to construct the new date via ZonedDateTime instead.

There is actually a unit test for this, but the MimeMessageBuilder adds a Date header by default if not specified. The test fix is to explicitly remove the Date header after building.