gistfile1.rb

require 'rexml/document'

module HashToXML
  def self.to_xml(any, **options)
    document = if any.is_a? REXML::Document
      any
    else
      to_xml_document(any, **options)
    end

    "".tap do |out|
      args = [options["indent"] || 2, options["transitive"], options["ie_hack"]]
      document.write(out, *args)
    end
  end

  def self.to_xml_document(any, **options)
    document = REXML::Document.new
    version  = options["version"]  || "1.0"
    encoding = options["encoding"] || "UTF-8"
    document << REXML::XMLDecl.new(version, encoding)

    convert(any, document)

    document
  end

private
  def self.convert(any, parent)
    case any
    when String
      parent.text = any
    when Hash
      any.each do |key, value|
        case key
        when '__attrs__', '__attributes__'
          attributes!(parent, value)
        when '__text__'
          parent.text = value
        else
          child = REXML::Element.new(key)
          convert(value, child)
          parent << child
        end
      end
    when Array
      attributes, *children = any
      attributes!(parent, attributes)
      children.each do |e|
        convert(e, parent)
      end
    else
      raise "Unsupported type: #{any.class}!"
    end
  end

  def self.attributes!(element, attrs)
    attrs.each { |k, v| element.add_attribute(k, v) }
  end
end

### TEST IT:

require 'json'

json = <<JSON
{
  "id": "atlassian-theme",
  "configuration": {
    "file_name": "org.codefirst.SimpleThemeDecorator.xml",
    "content": {
      "org.codefirst.SimpleThemeDecorator": [
        { "plugin": "[email protected]" },
        { 
          "cssUrl": "http://master.source.test.do/dist/theme.css",
          "jsUrl": "http://master.source.test.do/dist/theme.js"
        }
      ]
    }
  }
}
JSON

h = JSON.parse(json)

content = h["configuration"]["content"]

xml = HashToXML.to_xml(content)

puts xml

# # prints:
# <?xml version='1.0' encoding='UTF-8'?>
# <org.codefirst.SimpleThemeDecorator plugin='[email protected]'>
#   <cssUrl>
#     http://master.source.test.do/dist/theme.css
#   </cssUrl>
#   <jsUrl>
#     http://master.source.test.do/dist/theme.js
#   </jsUrl>
# </org.codefirst.SimpleThemeDecorator>