Fix references to undefined this

[astorije]

79eb83d introduces a couple uses of this in a function that doesn't know about the channel.

I preferred to pass the channel instead of making this part of the prototype (and therefore exporting it) but I can switch to that otherwise.

[xPaw]

@astorije I think prototype is better suited here (think private function in a class). Also, oops.

[astorije]

think private function in a class

What do you mean, I don't think it's possible to have a private member, is it?

[xPaw]

It is possible in an actual class, I just suggested that prototype is better here than passing references in the same file.

[astorije]

Actually, nothing .call() can't fix 😁
That way, still private, can still use this. What do you think?

It looks like private methods are an ES7 thing, and just not very pretty syntactic sugar over .call (same code, just this::writeUserLog(client, msg)).