def serialized_tag(tagname, str, additional_attributes = {}, &block)
      str << "<#{tagname} "
      serialized_attributes(str, additional_attributes)
      if block_given?
        str << '>'
        yield
        str << "</#{tagname}>"
      else
        str << '/>'
      end
    end
creates a XML tag with serialized attributes
@see SerializedAttributes#serialized_attributes