Provide builder for ResponseEntity

This commit introduces a ResponseEntityBuilder, which allows for
building of a ResponseEntity through a fluent API.

Issue: SPR-11752
Fix RestTemplate documentation for gzip encoding

Prior to this commit, RestTemplate's documentation advised to a
DecompressingHttpClient decorator along with Apache's HttpClient in
order to support gzipped responses.

Since this is now deprecated as of Apache HttpClient 4.3+, this commits
update the documentation with HttpClientBuilder.
Polish