Fix key and value in bench:set command so they're replicated

These fields weren't being replicated before (I noticed when I was
looking through the Raft log with less). This commit changes their names
from 'key' to 'Key' and 'value' to 'Value', which seems to fix it.
Merge pull request #1 from ongardie/master

Fix key and value in bench:set command so they're replicated