Elytra stops working on relog

As of 16w07a, one must jump in midair in order to activate the elytra. However, as a side effect of this, the elytra now stops working on world relog.

To reproduce:

1. create a new world
2. give yourself an elytra
3. put yourself in the Elytra flight mode
4. log out
5. log back in

you should no longer be flying.