make on_death compatible (return either player or nil)

api.lua

@@ -882,7 +882,14 @@ function mob_class:check_for_death(cmi_cause)
 	-- execute official engine on_death function if found
 	if self.on_death then
 
-		self:on_death(self, cmi_cause)
+		-- only return killer if punched by player
+		if cmi_cause.type == "punch" and is_player(cmi_cause.puncher) then
+			cmi_cause = cmi_cause.puncher
+		else
+			cmi_cause = nil
+		end
+
+		self:on_death(cmi_cause)
 
 		remove_mob(self, true)
 

api.txt

@@ -336,7 +336,8 @@ enhance mob functionality and have them do many interesting things:
                     is returned normal attack function continued.
    'on_die'         a function that is called when mob is killed (self, pos), also
                     has access to self.cause_of_death table.
-   'on_death'       Official engine version of above when mob killed (self, cause).
+   'on_death'       Official engine version of above when mob killed (self, killer),
+                    'killer' is only returned if player killed the mob.
    'on_flop'        function called when flying or swimmimng mob is no longer in
                     air/water, (self) paramater and return true to skip the built
                     in api flop feature.