QHttpServer: Add support for headers in QHttpServerResponse

Currently, we can only construct a QHttpServerResponse from the content type, the actual content and a status code. It would be very useful to be able to also set arbitrary HTTP-Headers for the response.

A basic use case would be content encoding: If we want to compress data before sending it, the Content-Encoding header must be set - which requires the application to set this header.

Note: I am aware that it is possible to specify headers via QHttpServerResponder. However, we cannot use this class directly when working with QHttpServer. I personally think this is simple and relevant enough to extend QHttpServerResponse to allow custom headers even when using the "simplified" interface.