Better empty response body support in RestTemplate

Prior to this change, RestTemplate returned an empty response body if:

* HTTP return status 204 or 304
* Content-length header equals 0

This change adds a new condition for this, better supporting RFC7230
section 3.4, for connections that are closed without response body:

* No Content-length header
* No Transfer-encoding: chunked header value
* a Connection: close header value

See SPR-7911 for previous efforts in that space.

Issue: SPR-8016