* 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
std::string method = base::ToUpperASCII(raw);
  if (method.empty() || method == "GET")
    return net::URLFetcher::GET;
  else if (method == "POST")
    return net::URLFetcher::POST;
  else if (method == "HEAD")
    return net::URLFetcher::HEAD;
  else if (method == "DELETE")
    return net::URLFetcher::DELETE_REQUEST;
  else if (method == "PUT")
    return net::URLFetcher::PUT;
  else if (method == "PATCH")
    return net::URLFetcher::PATCH;
  else  
    return net::URLFetcher::GET;
}
class ResponsePiper : public net::URLFetcherResponseWriter {
 public:
  explicit ResponsePiper(URLRequestFetchJob* job)
      : first_write_(true), job_(job) {}
  
  int Initialize(const net::CompletionCallback& callback) override {