SIP option to use SIP date format for due dates

New SIP configuration option "use_sip_date_format" which forces all SIP
dates to be formated using the SIP date format: YYYYMMDDZZZZHHMMSS

Without this setting, most dates already correctly use the SIP date
format.  However, for some historical reason, due dates are formatted as
ISO 8601 dates.  Enable this setting to force due dates to use the SIP
date format.

Signed-off-by: Bill Erickson <berick@esilibrary.com>
Signed-off-by: Jason Stephenson <jstephenson@mvlc.org>