TS-3978: Allow empty document caching to follow normal logic

[unixwitch]

No description provided.

[sudheerv]

Hmm..the requirement in the jira certainly seems reasonable, but, the current state of this is mainly to ensure ATS is not caching broken responses (connection failure in the middle of download).

Allowing to cache the empty docs only when Content-Length=0 indirectly ensures that the response was not broken. So, the patch must somehow ensure that the response is not broken, otherwise, there's a danger of caching invalid responses.

[zwoop]

I'm not sure that we're comfortable with this change, since it's been known to cause problems with empty docs in other areas (e.g. clustering). We'll have to take a close look at this, and decide if we should get this in for 7.0.0.

Also, the branch has now diverged, can you please rebase against current master?

[zwoop]

I'm going to close this for now, please open a new PR after we discuss this more?