It was in the game. It didn't light you on fire, though, it just did damage over time while you had them out. 6 damage per second iirc.
And it is on the wiki:
June 27, 2012 Patch (Pyromania Update)
Changed attributes:
Removed self-damage over time penalty
Heavy takes mini-crits while active and for 3 seconds after switching away.
Attack damage penalty reduced from -50% to -25%.