Follow-up fix for #5b711a

Not only do we need to preserve Mono in writeWith in the base class
but also in ReactorServerHttpResponse where buffers are mapped.

See gh-22800

spring-web/src/main/java/org/springframework/http/server/reactive/ReactorServerHttpResponse.java

@@ -122,7 +122,9 @@ class ReactorServerHttpResponse extends AbstractServerHttpResponse implements Ze
 	}
 
 	private Publisher<ByteBuf> toByteBufs(Publisher<? extends DataBuffer> dataBuffers) {
-		return Flux.from(dataBuffers).map(NettyDataBufferFactory::toByteBuf);
+		return dataBuffers instanceof Mono ?
+				Mono.from(dataBuffers).map(NettyDataBufferFactory::toByteBuf) :
+				Flux.from(dataBuffers).map(NettyDataBufferFactory::toByteBuf);
 	}
 
 }