def serialized_element_attributes(str='', additional_attributes=[], &block)
      attrs = self.class.xml_element_attributes + additional_attributes
      values = instance_values
      attrs.each do |attribute_name|
        value = values[attribute_name.to_s]
        next if value.nil?
        value = yield value if block_given?
        element_name = Axlsx.camel(attribute_name, false)
        str << "<#{element_name}>#{value}</#{element_name}>"
      end
      str
    end
serialized instance values at text nodes on a camelized element of the
attribute name. You may pass in a block for evaluation against non nil
values. We use an array for element attributes becuase misordering will
break the xml and 1.8.7 does not support ordered hashes.
@param [String] str The string instance to which serialized data is appended
@param [Array] additional_attributes An array of additional attribute names.
@return [String] The serialized output.