I believe your calculations are incorrect, professor monkey patch.

Fix faulty implementation of String#to_range.

```ruby
"10..50".to_range
# => 0..50
```

I think not.

Culprit: https://github.com/spree/spree_volume_pricing/blob/master/lib/spree_volume_pricing/engine.rb#L17-L20

For some inexplicable reason the to_range method is excluding the first character of the range start. "501" becomes "01" and "10" becomes "0".

Anyways I believe I've fixed it.