When serving image, return proper Last-Modified header

lib/travis/api/app/responders/image.rb

@@ -8,7 +8,7 @@ module Travis::Api::App::Responders
       headers['Pragma'] = "no-cache"
       headers['Expires'] = Time.now.utc.httpdate
       headers['Content-Disposition'] = %(inline; filename="#{File.basename(filename)}")
-      halt send_file(filename, type: :png)
+      halt send_file(filename, type: :png, last_modified: resource.last_build_finished_at)
     end
 
     private

spec/integration/v2/repositories_spec.rb

@@ -115,6 +115,7 @@ describe 'Repos' do
       Factory(:build, repository: repo, state: :passed, commit: on_bar)
       result = get('/repos/svenfuchs/minimal.png?branch=foo,bar', {}, headers)
       result.should deliver_result_image_for('passing')
+      result.headers['Last-Modified'].should == repo.last_build_finished_at.httpdate
     end
 
     it '"passing" when there is a running build but the previous one has passed' do