Explicitly support IPV6 masking

[yaauie]

Since the implementation that provides the IPV4_NETWORK method uses IPAddr#new and IPAddr#mask, both of which fully support IPV6, it is entirely possible to do IPV6 masking by using the IPV4_NETWORK method.

Add explicit support for IPV6 (whether by adding a IPV6_NETWORK that shares the implementation or otherwise, along with specs)

https://github.com/logstash-plugins/logstash-filter-fingerprint/blob/v3.2.1/lib/logstash/filters/fingerprint.rb#L149-L152