Prior to this commit, asking for a `Void` type using any of the
`ClientResponse#bodyTo*` methods would immediately return an empty
`Publisher` without consuming the response body.
Not doing so can lead to HTTP connection pool inconsistencies and/or
memory leaks, since:
* a connection that still has a response body being written to it cannot
be properly recycled in the connection pool
* incoming `DataBuffer` might not be released
This commit detects when `Void` types are asked as body types and in
those cases does the following:
1. Subscribe to the response body `Publisher` to allow the connection to
be returned to the connection pool
2. `cancel()` the body `Publisher` if the response body is not empty; in
that case, we choose to close the connection vs. consume the whole
response body
Those changes imply that `ClientHttpResponse` and other related
contracts don't need a `close()` method anymore.
Issue: SPR-16018
126ac849e5