* 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
#include "atom/browser/net/atom_url_request.h"
#include <string>
#include "atom/browser/api/atom_api_url_request.h"
#include "atom/browser/atom_browser_context.h"
#include "base/callback.h"
#include "content/public/browser/browser_thread.h"
#include "net/base/elements_upload_data_stream.h"
#include "net/base/io_buffer.h"
#include "net/base/load_flags.h"
#include "net/base/upload_bytes_element_reader.h"
#include "net/url_request/redirect_info.h"
namespace {
const int kBufferSize = 4096;
}  
namespace atom {
namespace internal {
class UploadOwnedIOBufferElementReader : public net::UploadBytesElementReader {