* 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
#include "atom/browser/net/url_request_fetch_job.h"
#include <algorithm>
#include <string>
#include "base/memory/ptr_util.h"
#include "base/strings/string_util.h"
#include "native_mate/dictionary.h"
#include "net/base/io_buffer.h"
#include "net/base/net_errors.h"
#include "net/http/http_response_headers.h"
#include "net/url_request/url_fetcher.h"
#include "net/url_request/url_fetcher_response_writer.h"
using content::BrowserThread;
namespace atom {
namespace {
net::URLFetcher::RequestType GetRequestType(const std::string& raw) {