Clarify that 416 is valid on a blob put

[sudo-bmitch]

Fixes #590.

[jcarter3]

I'll publicly log my opinion that while I agree this is a valid response code, I disagree that it's a MUST, especially given backwards compatibility with existing registries in production today. I'd rather push to say that in chunked upload, the final put should not have data at all.

[sudo-bmitch]

I'll publicly log my opinion that while I agree this is a valid response code, I disagree that it's a MUST, and would rather push to say that in chunked upload, the final put should not have data at all.

I think removing support for data in the final chunk is a bigger than this PR. There is language supporting it on the previous lines of the spec that have been there for a while. Can we make that a separate issue/PR?