CORS error in Safari

Hi Jeremy,

@perlman reported to me (and I reproduced) that Safari encounters a CORS error due to stricter CORS conditions than other browsers when using Igneous/CloudVolume `http://localhost:1337` which emits the headers:
```
Access-Control-Allow-Origin: *
Access-Control-Allow-Methods: GET, POST, OPTIONS
Access-Control-Allow-Headers: range
```

I tried adjusting the headers from CloudVolume, but I believe the error occurs before the request even reaches the server. Therefore, I think the error is blocked by the headers sent with the initial page load to https://neuroglancer-demo.appspot.com

If you have a moment, would you mind taking a look? I can examine Neuroglancer locally, but I'm not entirely sure how that particular site is configured.

Thanks!
Will