Default value missing in the output

[suleman-uzair]

To reproduce the issue, please review the provided code sample.

require "lutaml/model"
require 'lutaml/model/xml_adapter/ox_adapter'

Lutaml::Model::Config.configure do |config|
  config.xml_adapter = Lutaml::Model::XmlAdapter::OxAdapter
end

class Mstyle < Lutaml::Model::Serializable
  attribute :displaystyle, :string, default: -> { "true" }

  xml do
    root "mstyle"

    map_attribute :displaystyle, to: :displaystyle
  end
end

class Maths < Lutaml::Model::Serializable
  attribute :display, :string, default: -> { "block" }
  attribute :style, Mstyle, default: -> { Mstyle.new }

  xml do
    root "math"
    map_attribute :display, to: :display
    map_element :mstyle, to: :style
  end
end

xml = "<math xmlns=\"http://www.w3.org/1998/Math/MathML\" display='true'></math>"
Maths.from_xml(xml).to_xml

> "<math display=\"true\"/>\n"