I've noticed that a lot of our clients that use `demands` have the following `pre_send` functionality:

```python
if request.method in ('POST', 'PUT'):
    request.headers['Content-Type'] = 'application/json;charset=utf-8'
    request.data = json.dumps(request.data)
```

Maybe we can end up with moving it to `demands`?  By checking some `convert_to_json` flag (or any other better name)?

```python
self.pre_send(request_params):
    if request_params['method'] in ('POST', 'PUT') and request_params.get(`convert_to_json`):
        ....
```