QHttpServer: Content-Encoding support

The Content-Encoding header of a HTTP request is optional and can have several values. ( https://en.wikipedia.org/wiki/HTTP_compression ) While it is unusual to get a  non-multipart POST/PUT request that has one of these encodings, the area where Qt operates this is probably desirable to support with custom servers and clients. (e.g. JSON over bandwidth limited connections, pay-per-usage connections (cell networks) etc.)

Additionally, this header should be supported for normal HTTP multi-part parts as well. 

What is being requested is transparent decompression, such that if request body is written to an QIODevice, that what is written is the body with the decompression already applied. 

There should be a way to turn this off as well, deferring decompression until later.