RFC 7616 support

[bugeaud]

This would be interresting to broaden the supported hash to ensure compatibility toward RFC 7616.

Doing so, it would help us to use a standard secure auth mecanism and avoid app specific mechanism that are non-standard and can lead to bad implementations and this bad overall security.

[erikdubbelboer]

I had a quick look but nginx doesn't seem to come with any sha256 or sha512 implementation. Not sure which are best to use than without introducing extra dependencies.