#13 Request class: Add methods to send data in chunks

This is already part of #9:

Implement Request::send: Pass the (raw) data to the network handler.

This is already part of #9

No it isn't. The send method can only send a header, but no body. HTTP GET works, but HTTP POST or PUT will not work with the current code.

TODO:

• Add a RequestDataSource base class that defines methods for a data source that can deliver data to a Request object. That class can be used to send raw data over the network when used with the base Request class or the request class of another protocol that allows raw data.
• Add a HTTPRequestDataSource implementation that can be configured to output provided data in several formats used by HTTP requests (multipart/form-data, application/x-www-form-urlencoded, ...). The HTTPDataSource class shall allow selecting binary sources (files) and form parameters.
• Add the method Request::setDataSource that accepts a RequestDataSource instance.
• Extend the test for the Request class.
• ... and the HTTPRequest class.
• Write a test for the RequestDataSource
• ... and the HTTPRequestDataSource class.

TODO changed: RequestDataSource is now a base class for raw request data instead of just defining the interface.