Add a WebFlux integration test with a request body and delayed response

While upgrading to Undertow `2.3.18.Final`, we discovered a regression that is not visible with current Spring Framework test suite. This issue intends to add an additional test that triggers this regression. Notice for now, we stay on Undertow `2.3.17.Final` and I will create a related issue on https://issues.redhat.com/projects/UNDERTOW.