Prior to this commit, due to KT-5464 type inference issue there was not
proper way to provide body expectations with WebTestClient. This commit
provides a workaround by updating the existing Kotlin extension to
return a Kotlin compatible API.
Issue: SPR-15692
This commit removes the mention that Spring Boot 2.0 includes
Jackson Kotlin module since it is not the case as of M7, and
add a note about using bean validation annotations with Kotlin.
