Images using srcset attribute download both 1x and 2x versions in Safari and Firefox

When wrapping LazyLoad around an image that utilizes the `srcset` attribute, both the 1x and 2x versions download in Safari and Firefox. Chrome seems to do the right thing and only download one version.

```JS

```

According to the devtools, in Safari and Firefox both /path/to/image.png and /path/to/image@2x.png get download as you scroll past. I would expect on my Retina laptop only the 2x image to get downloaded (Chrome's behavior).