Bugs 2541 and 2587 - AddIssue must return date object as intended.

Bugs 2541 and 2587 - AddIssue must return date object as intended.

SIP actually relied on the AddIssue return that was not reliable.
AddRenew also updated to return C4::Dates object for datedue.

Please note, any running SIPServer will have to be restarted
*immediately* after applying this patch, because although Koha
C4 behaves as normal, the SIP server runs as a Net::Server with
components cached.  Changes will not be applied until SIPServer
restarts, and so checkout actions may fail until then.

Signed-off-by: Galen Charlton <galen.charlton@liblime.com>
Signed-off-by: Henri-Damien LAURENT <henridamien.laurent@biblibre.com>