* Electron version: 1.6.1
* Operating system: MacOS 10.12

Had a crash happen at random. Might want to glance at OnURLFetchComplete and see if there are any obvious bugs.

```
Thread 9 Crashed:: Chrome_IOThread
0   com.github.electron.framework 	0x000000010bdd3c40 0x10a237000 + 28953664
1   com.github.electron.framework 	0x000000010a2f5606 atom::URLRequestFetchJob::OnURLFetchComplete(net::URLFetcher const*) + 70
2   com.github.electron.framework 	0x000000010a2f5670 non-virtual thunk to atom::URLRequestFetchJob::OnURLFetchComplete(net::URLFetcher const*) + 16
3   com.github.electron.framework 	0x000000010a481d91 0x10a237000 + 2403729
4   com.github.electron.framework 	0x000000010a455c26 0x10a237000 + 2223142
5   com.github.electron.framework 	0x000000010a455efc 0x10a237000 + 2223868
6   com.github.electron.framework 	0x000000010a456243 0x10a237000 + 2224707
7   com.github.electron.framework 	0x000000010a458a75 0x10a237000 + 2234997
8   com.github.electron.framework 	0x000000010a45598e 0x10a237000 + 2222478
9   com.github.electron.framework 	0x000000010a46a4e3 0x10a237000 + 2307299
10  com.github.electron.framework 	0x000000010a6c6ef8 0x10a237000 + 4783864
11  com.github.electron.framework 	0x000000010a6c7086 0x10a237000 + 4784262
12  com.github.electron.framework 	0x000000010a4829f2 0x10a237000 + 2406898
13  com.github.electron.framework 	0x000000010a460577 0x10a237000 + 2266487
14  libsystem_pthread.dylib       	0x00007fffa1d55aab _pthread_body + 180
15  libsystem_pthread.dylib       	0x00007fffa1d559f7 _pthread_start + 286
16  libsystem_pthread.dylib       	0x00007fffa1d551fd thread_start + 13
```

Full dump: https://gist.github.com/pfrazee/a6fc2d2f9127892b9c4627953ebace4a
atom/browser/net/url_request_fetch_job.cc
net::URLFetcher::RequestType request_type;
  if (method.empty())
    request_type = GetRequestType(request()->method());
  else
    request_type = GetRequestType(method);
  fetcher_ = net::URLFetcher::Create(formated_url, request_type, this);
  fetcher_->SaveResponseWithWriter(base::WrapUnique(new ResponsePiper(this)));
  
  if (url_request_context_getter_)
    fetcher_->SetRequestContext(url_request_context_getter_.get());
  else
    fetcher_->SetRequestContext(request_context_getter());
  
  if (referrer.empty())
    fetcher_->SetReferrer(request()->referrer());
  else
    fetcher_->SetReferrer(referrer);
  
  if (upload_data && request_type == net::URLFetcher::POST) {
    std::string content_type, data;
    upload_data->GetString("contentType", &content_type);
    upload_data->GetString("data", &data);
    fetcher_->SetUploadData(content_type, data);
  }
  
  fetcher_->SetExtraRequestHeaders(
      request()->extra_request_headers().ToString());
  fetcher_->Start();
}
void URLRequestFetchJob::HeadersCompleted() {