Fix expirationDate property type

The object stored into NSUserDefaults for this property has type of NSDate.
Resolve crash with Twitter OAuth when user presses cancel
Merge pull request #755 from ay8s/task/fix-twitter-oauth-cancel

Resolve crash with Twitter OAuth when user presses cancel
Merge pull request #754 from denivip/fix-incompatible-types-warning

Fix expirationDate property type