Allow reading unmasked address when passed a CIDR string

This behavior caught me by surprise:

> ip = IPAddr.new('1.2.3.4/16')
=> #<IPAddr: IPv4:1.2.0.0/255.255.0.0>       
> ip.to_s
=> "1.2.0.0"           

Looking at the code I believe this is happening at https://github.com/ruby/ipaddr/blob/master/lib/ipaddr.rb#L649-L651, which is a very old commit, so this behavior is clearly expected. For my use case I need the host bits unmasked, so I want to parse 1.2.3.4/16 and be able to read back something like ip.address as 1.2.3.4 and the netmask as 255.255.0.0. It looks like that's not currently possible with this gem, which surprises me. Am I overlooking a method, or is there a nuance in the CIDR spec implemented here I've forgotten?

lib/ipaddr.rb の 649-651 行目から始め、CIDR 文字列がどのように解析されて格納されるかを追跡します。現在のアドレスとネットマスクのアクセサを要求されている動作と比較し、1.2.3.4/16 についてマスクされていないアドレスとネットマスクの両方を読み戻せるようにする API 変更を定義します。