* Electron version: 1.4.5
* Operating system: Win 10 x64

### Expected behavior

`net.request` sends cookie as what I set

### Actual behavior

`net.request` sends cookie as default session's cookie

### How to reproduce

```
const {net} = require('electron');
const request = net.request({
	method: 'GET',
	url: 'https://mydomain.com/foo',
	session: null
});
request.removeHeader('Cookie');
// If current default session has a cookie `foo=11111`, 
// `request.setHeader` here has no effect. the request always send `foo=11111` 
// but expected is `foo=22222`
request.setHeader('Cookie', 'foo=22222');
request.end();
```

atom/browser/net/atom_url_request.cc
#include "atom/browser/net/atom_url_request.h"
#include <string>
#include "atom/browser/api/atom_api_url_request.h"
#include "atom/browser/atom_browser_context.h"
#include "base/callback.h"
#include "content/public/browser/browser_thread.h"
#include "net/base/elements_upload_data_stream.h"
#include "net/base/io_buffer.h"
#include "net/base/upload_bytes_element_reader.h"
namespace {
const int kBufferSize = 4096;
}  
namespace atom {
namespace internal {
class UploadOwnedIOBufferElementReader : public net::UploadBytesElementReader {