Do not attempt to destroy not persisted session

Otherwise Sequel is raising error like this:

(Sequel::NoExistingObject) "Attempt to delete object did not result in a single row modification (Rows Deleted: 0, SQL: DELETE FROM \"sessions\" WHERE (\"id\" IS NULL))"
Merge pull request #72 from andreychernih/fix-new-session-deletion

Do not attempt to destroy not persisted session