Sneak-flying doesn't work

When pressing the sneak key and the fly up key at the same time, the player hovers in the air as expected, but still travels at normal flying speed.

To reproduce:

1. create a new creative world
2. enter fly mode
3. fly around. note the flying speed
4. press the fly up key and sneak key at the same time
5. fly around. note that flying speed is the same as if the two keys aren't held