Fix set cookie every request

Without this patch, the response will always set cookie, even the session is not changed.

For example:

```py
@app.route('/')
def home():
    return 'home'


@app.route('/session')
def set_session():
    session['foo'] = 'bar'
    return 'session'
```

If you have visited `/session`, the next time when you visit `/` it will always send the `Set-Cookie` header.