Setting a gamerule with upper case letters crashes Minecraft

Release_466466

Summary:
Issuing any valid gamerule with upper case letters causes Minecraft to crash. Examples:

• /gamerule doMobSpawning true
• /gamerule PVP false

Impact:
Game crashes

Repro Steps:

1. Load any world
2. Issue command /gamerule doMobSpawning false

Observed Results:
Game crashes

Expected Results:
Changes gamerule (or at least throws up an error)

Screenshots/videos attached:
n/a