Nokogiri for Cucumber steps that need to compare HTML attributes

I have found on numerous occasions that I need to look at HTML attributes when diffing tables etc. in Cucumber. My current need for this arrises as I have a gallery of thumbnails that all have alt tags who’s values I need to compare as there is not a text equivalent.

A feature statement like this:

Then I should see the following thumbnails
  | Mona Lisa  |
  | Sunflowers |

Could be achieved with the following step:

Then /^I should see the following thumbnails$/ do |table|
  nodes = Nokogiri::HTML(response.body).css('ul#gallery li img')
  table.diff!( { |img| [img.attributes['alt'].to_s] })