...because I'm really not clear why this pair of lines isn't written:
<pre>
if command
    instance.public_send(body['message'], command, *args)
else
    instance.public_send(body['message'], *args)
end
</pre>

That seems like it avoids excess mutation of the array, the expense of shuffling all the elements around in memory, and make *much* clearer what is going on around command handling to my eyes.  I can now see in an instant that, for example, command is only sometimes present, and is injected as the first argument when present, and ignored entirely when absent.

That seems like it avoids a whole shed-load of messing around.