allow sending data in request body

This allows providers to pass string data to ``getContent``. If a string is passed it gets sent as request body rather than through request parameters.

This allows integrating APIs that expect a client to send the message payload as a request body.

A provider that implements such an API may be found at [hairmare/SmsSender-Swisscom-Provider](https://github.com/hairmare/SmsSender-Swisscom-Provider). I'll create a pull-request for the provider when the underlying service is considered stable.