ruby Array#reduce with an initial value

ruby_reduce.rb

class Array
  def reduce(func, init)
    if self.length < func.arity
      res = nil
    elsif self.length == func.arity
      res = func.call(*self)
    else
      res = func.call(self[0], self[1..-1].reduce(func, nil))
    end
    init.nil? ? res : func.call(init, res) 
  end
end
Test.assert_equals( ['a','y','!'].reduce(->(x,y){x+y}, 'y'), 'yay!')
Test.assert_equals( ['1','2','3'].reduce(->(x,y){x+y}, '0'), '0123')