Space + Shift while flying no longer locks you in place on y axis

Release_478003

Summary:
While flying in creative mode, holding space and shift at the same time no longer locks you in place. You instead sink at a slower than normal rate.

Impact:
Placing blocks against blocks with right click interactions while flying is now much more difficult. This is particularly useful while placing command blocks. This bug will not break any maps, it's just inconvenient for developing them.

Repro Steps:
1. Create a new world
2. Enter creative mode
3. Fly
4. Hold shift and space at the same time.

Observed results:
Your player sinks.

Expected results:
You should be locked on your y-coordinate.

Screenshots/videos attached:
No