[0.85] minor fix


git-svn-id: https://forge.indepnet.net/svn/glpi/branches/0.85-work@21283 35612d4c-9d10-0410-837c-bf9dd1be60d7
[0.85] Fix due_date mandatory on sla set


git-svn-id: https://forge.indepnet.net/svn/glpi/branches/0.85-work@21285 35612d4c-9d10-0410-837c-bf9dd1be60d7