* Electron version: 1.4.9
* Operating system: Windows 10 x64

### Expected behavior
The new request generated by interceptHttpProtocol should work normally with new session(slaveSession).

### Actual behavior
The new request seems to stick with the old session(mainSession).

### How to reproduce
```
const electron = require('electron')
const {app, protocol} = require('electron')
const BrowserWindow = electron.BrowserWindow
const {session} = require('electron')

let mainWindow
let mainSession
let slaveSession

app.on('ready', function(){
  mainSession = session.fromPartition('main')
  slaveSession = session.fromPartition('slave')
  mainWindow = new BrowserWindow({
    webPreferences: {
        javascript: true,
        plugins: true,
        webSecurity: false,
        session: mainSession
      }
    });
  
  mainWindow.webContents.openDevTools()

  mainSession.protocol.interceptHttpProtocol('http', function(request, callback) {
    if(request.url.indexOf('/api/list') >= 0){
      request.url = "custom:" + request.url.split(':')[1]
      callback(request)
    } else {
      console.log('pass ' + request.url)
      callback({'url': request.url, 'method': request.method, 'session': slaveSession})  
    }
  }, function(error){
    if (error) log("interceptHttpProtocol error: " + error)
  })

  mainWindow.loadURL("http://pan.baidu.com")
})
```
The following codes will enter infinite loop:
```
pass http://pan.baidu.com/
pass http://pan.baidu.com/
...
...
```
But there is no interceptHttpProtocol defined on slaveSession, is it a bug?

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 {