Move RxNetty support to test scope

The RxNetty runtime support is not meant to be officially supported and
should be restricted to testing purposes only.

Issue: SPR-15444

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

@@ -5,6 +5,6 @@
  * {@link org.springframework.http.server.reactive.HttpHandler} for processing.
  *
  * <p>Also provides implementations adapting to different runtimes
- * including Servlet 3.1 containers, Netty + Reactor IO or RxNetty, and Undertow.
+ * including Servlet 3.1 containers, Netty + Reactor IO, and Undertow.
  */
 package org.springframework.http.server.reactive;

spring-web/src/test/java/org/springframework/http/server/reactive/RxNettyHttpHandlerAdapter.java

@@ -36,6 +36,7 @@ import rx.RxReactiveStreams;
 
 /**
  * Adapt {@link HttpHandler} to the RxNetty {@link RequestHandler}.
+ * For internal use within the framework.
  *
  * @author Rossen Stoyanchev
  * @since 5.0

spring-web/src/test/java/org/springframework/http/server/reactive/RxNettyServerHttpRequest.java

@@ -39,6 +39,7 @@ import org.springframework.util.MultiValueMap;
 
 /**
  * Adapt {@link ServerHttpRequest} to the RxNetty {@link HttpServerRequest}.
+ * For internal use within the framework.
  *
  * @author Rossen Stoyanchev
  * @author Stephane Maldini

spring-web/src/test/java/org/springframework/http/server/reactive/RxNettyServerHttpResponse.java

@@ -38,7 +38,8 @@ import org.springframework.util.Assert;
 
 /**
  * Adapt {@link ServerHttpResponse} to the RxNetty {@link HttpServerResponse}.
- *
+ * For internal use within the framework.
+ * 
  * @author Rossen Stoyanchev
  * @author Stephane Maldini
  * @author Sebastien Deleuze