fixed broken shuffle for arrays of length 1 or 0

Coffeescript's range evaluator (in this case [a.length-1..1]) produces inappropriate results for Fisher-Yates when the array is length 1 or 0. The unfixed shuffle pushes undefined values onto the the array. A length check is necessary to continue with this style.
How do you feel about `return a unless a.length >= 2`. it saves and indent and might look cleaner.