Digest in requests with no payload body

[ioggstream]

I expect

An example of using Digest in a request with no payload body.
There is a misconceptions that Digest does not apply to request with no payload body.

As it's not a special case #849 (comment) for sha-256 we can just use

In [120]: sha256(b'').hexdigest()                                                                                                                                                                                                            
Out[120]: 'e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855'

Other algorithms are free to define a different computation, and the examples will only be tied to sha-256