Remove nullabe designation from DueAt. It causes a build error, and is unnecessary as string is a nullable type.
Remove nullabe designation from DueAt. It causes a build error, and is unnecessary as string is a nullable type.

dcrowe/ZendeskApi_v2,mwarger/ZendeskApi_v2,mattnis/ZendeskApi_v2,CKCobra/ZendeskApi_v2