Make callbacks use correct number of arguments

[bruse]

In the functions where 'fs' gives a callback of the form function(err),
mock-fs will give a callback of the form function(err, undefined). This
is not equivalent, and will mean using mock-fs functions in place like
some of async.js flow control functions will cause a different behaviour
than the native 'fs' functions.

See https://gist.github.com/bruse/5e5ba24bc5109090f75b for an example
of this inconsistent behavior.

This is one way of fixing it. If we are always sure that the functions that
are supposed to return data never returns undefined, we can make it a
lot shorter I suppose, but I chose to be explicit.

What do you think?

[tschaub]

Thanks for the report @bruse. I like that your changes here explicitly force the callback arity, but I think the changes in #11 might be easier to maintain. Let me know if you think that will suffice or if it misses some cases.

[tschaub]

Another alternative would be to add callback arguments when calling maybeCallback. E.g.

  return maybeCallback(callback, this, function(err, exists) {
  // ...
  };

And then maybeCallback could check func.length. This would be equivalent to your change.

I'm inclined to start with the changes in #11 and then make a more involved change if another issue comes up.

[bruse]

Sounds good. I'm not very familiar with your code base, but if you're confident that the functions related to the cases where we actually expect two arguments never return undefined, this indeed looks better.