When killing the mob being spectated, specifically with /kill @e[type=Mob, you cannot leave spectator mode until you close and reopen the world.

What I expected to happen was...:
Typing /gamemode [Mode] would change to that gamemode.

What actually happened was...:
Typing /gamemode [Mode] does not change your gamemode, but shows the full HUD Except for the health and hunger bars (In survival), and only your hand or item (in creative).

Steps to Reproduce:
1. Find a mob, enter spectate mode, and spectate the mob.
2. Kill the mob with /kill @e[type=Mob].
3. Try to leave spectator mode by typing /gamemode 0, 1, or 2.