* 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_job_factory.cc
#include "atom/browser/net/atom_url_request_job_factory.h"
#include "base/memory/ptr_util.h"
#include "base/stl_util.h"
#include "content/public/browser/browser_thread.h"
#include "net/base/load_flags.h"
#include "net/url_request/url_request.h"
using content::BrowserThread;
namespace atom {
typedef net::URLRequestJobFactory::ProtocolHandler ProtocolHandler;
AtomURLRequestJobFactory::AtomURLRequestJobFactory() {}