use the original name as the key in the doc

Use the original name as the key in the doc when :name is redifined.

    class Article
      include Representable::JSON
      attr_accessor :title
      property :heading, :name => :title # implicit :from => :heading
    end

    a = Article.new; a.title = "Godzilla!"
    a.to_json # => '{"heading" : "Godzilla!"}'