* 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/api/atom_api_protocol.h
#ifndef ATOM_BROWSER_API_ATOM_API_PROTOCOL_H_
#define ATOM_BROWSER_API_ATOM_API_PROTOCOL_H_
#include <map>
#include <string>
#include <vector>
#include "atom/browser/api/trackable_object.h"
#include "atom/browser/atom_browser_context.h"
#include "atom/browser/net/atom_url_request_job_factory.h"
#include "base/callback.h"
#include "base/memory/weak_ptr.h"
#include "content/public/browser/browser_thread.h"
#include "native_mate/arguments.h"
#include "native_mate/dictionary.h"
#include "native_mate/handle.h"
#include "net/url_request/url_request_context.h"
namespace base {
class DictionaryValue;
}
namespace atom {
namespace api {
std::vector<std::string> GetStandardSchemes();
void RegisterStandardSchemes(const std::vector<std::string>& schemes,
                             mate::Arguments* args);
class Protocol : public mate::TrackableObject<Protocol> {