Include original canonical image src after rewriting

Author: josh

lib/html/pipeline/camo_filter.rb

@@ -37,6 +37,7 @@ def call
           next if asset_hostname_whitelisted?(uri.hostname)
 
           element['src'] = asset_proxy_url(uri.to_s)
+          element['data-canonical-src'] = uri.to_s
         end
         doc
       end

test/html/pipeline/camo_filter_test.rb

@@ -17,6 +17,8 @@ def test_camouflaging_http_image_urls
     orig = %(<p><img src="http://twitter.com/img.png"></p>)
     assert_includes 'img src="' + @asset_proxy_url,
       CamoFilter.call(orig, @options).to_s
+    assert_includes 'data-canonical-src="http://twitter.com/img.png"',
+      CamoFilter.call(orig, @options).to_s
   end
 
   def test_doesnt_rewrite_dotcom_image_urls