This commit makes changes to WebClient and WebTestClient in oder to
limit setting the body according to HTTP method and also to facilitate
providing the request body as Object.
Specifically, this commit:
- Moves methods that operate on the request body to a RequestBodySpec
in both WebClient and WebTestClient, and rename them to `body`.
These methods now just *set* the body, without performing
an exchange (which now requires an explicit exchange call).
- Parameterizes UriSpec in both WebClient and WebTestClient, so that
it returns either a RequestHeadersSpec or a RequestBodySpec.
Issue: SPR-15394
118f33aeda