I think `within_range?` is confusing when used with a method that has kw args.  It may or may not accept 2 args depending on what they are:

``` irb
irb(main):001:0> def arity_kw(x, y:1, z:2); end
=> nil
irb(main):002:0> arity_kw(nil, nil)
ArgumentError: wrong number of arguments (2 for 1)
	from (irb):1:in `arity_kw'
	from (irb):2
	from /Users/myron/.rubies/ruby-2.0.0-p247/bin/irb:12:in `<main>'
irb(main):003:0> arity_kw(nil, {})
=> nil
```

I think we're better of testing these methods with an actual list of args rather than using a helper where you pass a number since the values are important.