Spring 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.

Comment From: sdeleuze

See related issue https://issues.redhat.com/browse/UNDERTOW-2512.