Crystal: Allow `record` to support default values when using *::Serializable

[Blacksmoke16]

https://play.crystal-lang.org/#/r/8u3w

require "json"
require "yaml"

yaml = <<-YAML
---
id: 1
YAML

json = <<-JSON
{"id":1}
JSON

record Test, id : Int32, active : Bool? = false do
  include JSON::Serializable
  include YAML::Serializable
end


pp Test.from_yaml yaml # => Test(@active=nil, @id=1)
pp Test.from_json json # => Test(@active=nil, @id=1)

I would expect this to set active to false if the property is missing, however it gets set to nil. Similarly, if the type of the property is not nillable, an exception saying the property is missing is raised versus just using the default value.

[RX14]

It does respect default values, just that record macro sets these default values in the constructor, not in the getter definitions, so they do not get "picked up" by ::Serializable.

So this is just a record feature request.