Update Mono.then(Supplier) deprecated calls

Reactor Core has now deprecated the `Mono.then(Supplier)`.
This is now replaced with `Mono.then(Mono.defer(Supplier))`.

spring-test/src/main/java/org/springframework/test/web/reactive/server/FluxExchangeResult.java

@@ -90,7 +90,7 @@ public class FluxExchangeResult<T> extends ExchangeResult {
 	public byte[] getResponseBodyContent() {
 		return this.body.ignoreElements()
 				.timeout(this.timeout, Mono.error(TIMEOUT_ERROR))
-				.then(() -> Mono.just(super.getResponseBodyContent()))
+				.then(Mono.defer(() -> Mono.just(super.getResponseBodyContent())))
 				.block();
 	}