* Electron version: 1.8.2-beta.3
* Operating system: MacOS 10.12

### Expected behavior

File downloads from custom protocols should complete without issue.

### Actual behavior

Before even triggering the custom protocol handler, the download-item emits 'done' with a state of 'interrupted'. In my demo linked below, I use the new registerStreamProtocol, but I was able to reproduce using registerHttpProtocol as well.

### How to reproduce

Use https://github.com/pfrazee/electron-download-interrupt-bug

atom/browser/net/atom_url_request.cc
scoped_refptr<net::HttpResponseHeaders> response_headers) const {
  DCHECK_CURRENTLY_ON(content::BrowserThread::UI);
  if (delegate_)
    delegate_->OnResponseStarted(response_headers);
}
void AtomURLRequest::InformDelegateResponseData(
    scoped_refptr<net::IOBufferWithSize> data) const {
  DCHECK_CURRENTLY_ON(content::BrowserThread::UI);
  
  
  if (delegate_)
    delegate_->OnResponseData(data);
}
void AtomURLRequest::InformDelegateResponseCompleted() const {
  DCHECK_CURRENTLY_ON(content::BrowserThread::UI);
  if (delegate_)
    delegate_->OnResponseCompleted();
}
void AtomURLRequest::InformDelegateErrorOccured(const std::string& error,
                                                bool isRequestError) const {
  DCHECK_CURRENTLY_ON(content::BrowserThread::UI);
  if (delegate_)
    delegate_->OnError(error, isRequestError);
}
}