IP#is_in? fails testing subnets

When testing if a scope is included in another one, #is_in? fails :

```
>> net = IP.new("172.22.5.64/28")
=> #<IP::V4 172.22.5.64/28>
>> sup = IP.new("172.22.0.0/16")
=> #<IP::V4 172.22.0.0/16>
>> net.is_in?(sup)
=> false
```
