MockMvc re-uses response instance on async dispatch

MockMvc now properly detects the presence of an AsyncContext and
re-uses the response instance used to start it.

This commit also includes a minor fix in
ResponseBodyEmitterReturnValueHandler to ensure it does not disable
ETag related content buffering for reactive return values that do not
result in streaming (e.g. single value or collections).

Issue: SPR-16067

spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/method/annotation/ResponseBodyEmitterReturnValueHandler.java

@@ -140,23 +140,23 @@ public class ResponseBodyEmitterReturnValueHandler implements HandlerMethodRetur
 
 		ServletRequest request = webRequest.getNativeRequest(ServletRequest.class);
 		Assert.state(request != null, "No ServletRequest");
-		ShallowEtagHeaderFilter.disableContentCaching(request);
 
 		ResponseBodyEmitter emitter;
-
 		if (returnValue instanceof ResponseBodyEmitter) {
 			emitter = (ResponseBodyEmitter) returnValue;
 		}
 		else {
 			emitter = this.reactiveHandler.handleValue(returnValue, returnType, mavContainer, webRequest);
+			if (emitter == null) {
+				// Not streaming..
+				return;
+			}
 		}
-
-		if (emitter == null) {
-			return;
-		}
-
 		emitter.extendResponse(outputMessage);
 
+		// At this point we know we're streaming..
+		ShallowEtagHeaderFilter.disableContentCaching(request);
+
 		// Commit the response and wrap to ignore further header changes
 		outputMessage.getBody();
 		outputMessage.flush();