def attribute_value(attribute_name)
      attribute_name = attribute_name.to_s.tr('_', '-') if attribute_name.is_a?(::Symbol)
      element_call { @element.attribute attribute_name }
    end
def attribute_value(attribute_name)
      attribute_name = attribute_name.to_s.tr('_', '-') if attribute_name.is_a?(::Symbol)
      element_call { @element.attribute attribute_name }
    end
Returns given attribute value of element.

 @example
   browser.a(id: "link_2").attribute_value "title"
   #=> "link_title_2"

 @param [String, ::Symbol] attribute_name
 @return [String, nil]