[Minor] Skip modifying headers for skipped tasks

lualib/lua_mime.lua

@@ -515,6 +515,10 @@ end
 -- Adds/removes headers both internal and in the milter reply
 --]]
 exports.modify_headers = function(task, hdr_alterations)
+  if task:has_flag('skip') then
+    -- Cannot set flags for skipped (and unprocessed) task
+    return
+  end
   local add = hdr_alterations.add or {}
   local remove = hdr_alterations.remove or {}