auto merge of #3934 : guillaumebort/servo/fix/84, r=jdm

Actually `<img>` elements in `<noscript>` are not prefetched anymore.

Probably because html5ever already parses the `<noscript>` content as raw
text data if `scripting_enabled` is activated.

See https://github.com/servo/html5ever/blob/servo/src/tree_builder/rules.rs#L126

Also, added a test to the images cache.

components/script/dom/htmlimageelement.rs

@@ -60,9 +60,6 @@ impl<'a> PrivateHTMLImageElementHelpers for JSRef<'a, HTMLImageElement> {
 
                 // inform the image cache to load this, but don't store a
                 // handle.
-                //
-                // TODO (Issue #84): don't prefetch if we are within a
-                // <noscript> tag.
                 image_cache.send(image_cache_task::Prefetch(img_url));
             }
         }