Death messages from mobs are grammatically incorrect

Identical to MC-129907.

When killed by an unnamed mob, the death message displays as
"<username> was slain by <Mob>"
One would expect this to look like one of these instead:
"<username> was slain by a <Mob>"
"<username> was slain by a <mob>"
Named mobs, as well as other players, produce grammatically correct messages.