Would recommend handling an empty set

```
irb(main):005:0> ZeroFill.new([])
NoMethodError: undefined method `values' for nil:NilClass
	from zero_fill.rb:32:in `detect_input_type'
	from zero_fill.rb:6:in `initialize'
	from (irb):5:in `new'
	from (irb):5
	from /Users/onebert/.rbenv/versions/1.9.3-p194/bin/irb:12:in `<main>'
```