Leave "Date" header alone.

Leave "Date" header alone.

Prior to this change, Stump removed the supplied Date: header, and
replaced it with one of its own, which reflected the time at which the
article was approved. Following discussion in unnm, it seemed clear
that people would like the Date: header to be unmolested.

Signed-off-by: Matthew Vernon <matthewv@chiark.greenend.org.uk>