Callback will not get invoked when non SSL connection is blocked

When a script tries to access a non-SSL XHR from an https page it gets blocked by the browser:

```
[blocked] The page at 'https://xxxx.com/' was loaded over HTTPS, but ran insecure content from 'http://yyyy.com/': this content should also be loaded over HTTPS.
```

When this happens the `end()` callback is never invoked.