Makes a POST request, setting Authorization
        and Content-Type headers by default

def _post_json(self, url, data, **kw):
        '''
        Makes a POST request, setting Authorization
        and Content-Type headers by default
        '''
        data = json.dumps(data)
        headers = kw.pop('headers', {})
        headers.setdefault('Content-Type', 'application/json')
        headers.setdefault('Accept', 'application/json')

        kw['headers'] = headers
        kw['data'] = data
        return self._post(url, **kw)