Fix bug with passing in Fixnum argument

I noticed the discussion in PR #4 about making sure the updates didn't break existing code that uses this gem; but when passing in Fixnum arguments to, say, `get_users`, the code does break. So I've added a check for Fixnum before assuming it's a hash.

Ultimately, this (and a number of other things, like the `response_format` handling) needs to be refactored. For example, I'm not sure we need a splat for `args`; why not just use the singular name `arg` and pass in a string, a number, or a hash?