Package private request and response implementations

Concrete server and client, reactive request and response
implementations should not have to be accessed outside their package.
They could be seen as private to their HttpHandler adapters and
ClientHttpConnector's respectively.

The one exception, WebSocket upgrades in spring-webflux, is an internal
framework use case, accommodated via downcast + accessors in the
abstract base classes.

spring-test/src/main/java/org/springframework/mock/http/server/reactive/MockServerHttpRequest.java

@@ -103,6 +103,10 @@ public class MockServerHttpRequest extends AbstractServerHttpRequest {
 		return this.cookies;
 	}
 
+	@Override
+	public <T> T getNativeRequest() {
+		throw new IllegalStateException("This is a mock. No running server, no native request.");
+	}
 
 	/**
 	 * Shortcut to wrap the request with a {@code MockServerWebExchange}.

spring-test/src/main/java/org/springframework/mock/http/server/reactive/MockServerHttpResponse.java

@@ -76,6 +76,11 @@ public class MockServerHttpResponse extends AbstractServerHttpResponse {
 		this.writeHandler = writeHandler;
 	}
 
+	@Override
+	public <T> T getNativeResponse() {
+		throw new IllegalStateException("This is a mock. No running server, no native response.");
+	}
+
 
 	@Override
 	protected void applyStatusCode() {