MINOR: samples: add the http_date([<offset>]) sample converter.

MINOR: samples: add the http_date([<offset>]) sample converter.

Converts an integer supposed to contain a date since epoch to
a string representing this date in a format suitable for use
in HTTP header fields. If an offset value is specified, then
it is a number of seconds that is added to the date before the
conversion is operated. This is particularly useful to emit
Date header fields, Expires values in responses when combined
with a positive offset, or Last-Modified values when the
offset is negative.