Player Chat Event setFormat breaks when % is in message (Fixes Bukkit-5307)

The Issue:
String.format throws an IllegalFormatException when a % is in the message that is being formatted.

Justification for this PR:
This is a cheap workaround which fixes this bug.

https://bukkit.atlassian.net/browse/BUKKIT-5307
Relevant: http://stackoverflow.com/q/7429852/3106527