Add IPAddr#to_json/as_json

[taketo1113]

Add to_json/as_json methods to get json format string.

When use json gem, IPAddr#to_json remove prefix address with CIDR.
Although json gem have no paln to support for IPAddr class.
ruby/json#503

Actual behavior

require 'ipaddr'
require 'json'

IPAddr.new('172.16.0.0/24').to_json #=> "\"172.16.0.0\""

This PR behavior

IPAddr#to_json method return json format address wit prefix without json gem.
(When to require json gem, it have same behavior.)

require 'ipaddr'

IPAddr.new('172.16.0.0/24').to_json #=> "\"172.16.0.0/24\""

IPAddr.new('172.16.0.1').to_json #=> "\"172.16.0.1\""

[knu]

Was your intent as_json(*args) because you gave it a name?

[taketo1113]

@knu It is not necessary a name. I have removed a name of args.
It is from another patch which is ruby/json@f1ffc4a

[knu]

I prefer the implementation of to_json to be as_json.to_json(*args) so it is easier to change how IPAddrs are encoded simply by overriding as_json. With the current definition, when you want it to be def as_json(*) = to_f you'll need to rewrite to_json as well.

[knu]

Forget about to_f, I think it mistook this for something else. In any case, I like how JSON::GenericObject defines to_json. I'll change it as appropriate later. Thanks for contributing!

[knu]

The json library defines the generic to_json in Object, so we may not have to implement one in IPAddr.
Should we provide to_json so you can use it without the json library?

[taketo1113]

@knu

The json library defines the generic to_json in Object, so we may not have to implement one in IPAddr. Should we provide to_json so you can use it without the json library?

Because when use to_json method of json gem, return json format string without prefix.
It is unable to serialize/deserialize IPAddr/CIDR value.

Expected behavior

require 'ipaddr'
require 'json'

IPAddr.new('172.16.0.0/24').to_json #=> "\"172.16.0.0/24\""

Actual behavior

require 'ipaddr'
require 'json'

IPAddr.new('172.16.0.0/24').to_json #=> "\"172.16.0.0\""

ruby/json#503