* 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
DCHECK_CURRENTLY_ON(content::BrowserThread::IO);
  if (!request_) {
    return;
  }
  request_->RemoveRequestHeaderByName(name);
}
void AtomURLRequest::DoSetAuth(const base::string16& username,
                               const base::string16& password) const {
  DCHECK_CURRENTLY_ON(content::BrowserThread::IO);
  if (!request_) {
    return;
  }
  request_->SetAuth(net::AuthCredentials(username, password));
}
void AtomURLRequest::DoCancelAuth() const {
  DCHECK_CURRENTLY_ON(content::BrowserThread::IO);
  if (!request_) {
    return;
  }
  request_->CancelAuth();
}
void AtomURLRequest::DoCancelWithError(const std::string& error,
                                       bool isRequestError) {
  DoCancel();
  content::BrowserThread::PostTask(
      content::BrowserThread::UI, FROM_HERE,
      base::Bind(&AtomURLRequest::InformDelegateErrorOccured, this, error,
                 isRequestError));
}
void AtomURLRequest::DoSetLoadFlags(int flags) const {