* 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
if (!options->IsType(base::Value::Type::DICTIONARY)) {
    NotifyStartError(net::URLRequestStatus(
          net::URLRequestStatus::FAILED, net::ERR_NOT_IMPLEMENTED));
    return;
  }
  std::string url, method, referrer;
  base::DictionaryValue* upload_data = nullptr;
  base::DictionaryValue* dict =
      static_cast<base::DictionaryValue*>(options.get());
  dict->GetString("url", &url);
  dict->GetString("method", &method);
  dict->GetString("referrer", &referrer);
  dict->GetDictionary("uploadData", &upload_data);
  
  GURL formated_url(url);
  if (!formated_url.is_valid()) {
    NotifyStartError(net::URLRequestStatus(
          net::URLRequestStatus::FAILED, net::ERR_INVALID_URL));
    return;
  }