/gamerule will crash the game if the first argument contains uppercase letters

The bug

When the player tries to use the /gamerule command where the first argument contains any uppercase letters, the game will crash.

How to reproduce

1. Start a new world with cheats/commands enabled
2. Run the command /gamerule doDaylightCycle [true/false] (Note that this will work with any /gamerule command, as long as the first argument contains uppercase letters. It does not specifically need to be doDaylightCycle.)