Fixes 0011015: mc_issue_update erases 'Due date' field

The due_date field has been added to the WSDL and is now exposed
by mc_issue_get and accepted by mc_issue_add and mc_issue_update.
Proper access checkes are in place.

The due_date is read with the local time zone offset applied since
nusoap seems to apply the offset wrong when parsing.

Reference: http://www.nabble.com/NUSOAP-datetime-UTC-issue-td16627310.html

Tests have been added for both adding and updating an issue, skipped
if the thresholds don't allow the admin user to view and update the
due date.