This commit makes a change to WebClient in oder to facilitate getting
the response body as a `Mono<Object>` or `Flux<Object>` without having
to deal with `ClientResponse`.
Specifically, this commit:
- Adds `RequestHeaderSpec.retrieve` methods, next to `exchange`, that
return the response body (and not a `ClientResponse`). Two convenience
methods return the response body as `Mono` or `Flux`.
- Adds ClientResponse.toRequestEntity to convert the ClientResponse
into a RequestEntity.
Issue: SPR-15294
e6b4edc757