Reconsider if zero-length body is invalid

[dashpole]

I'm opening this issue to track #543 (comment), so it doesn't fall through the cracks. It is blocking kubernetes/kubernetes#128919 (comment).

From @liggitt:

Is an explicitly zero-length body always invalid? If not, the merged PR for this looks incorrect at first glance...

Things like https://lists.w3.org/Archives/Public/ietf-http-wg/2010JulSep/0276.html led me to think a zero-length body on put/post/patch was valid

How else would you send a text/plain POST or PUT intended to set an empty value?

(we spotted this while reviewing an update of the library for Kubernetes in kubernetes/kubernetes#128919)

Thanks!

[emicklei]

@dashpole thank you for this reminder. I will dive back into this.

[emicklei]

@dashpole can you have a look at this #581