Refine tests for SPR-14066

Explicitly define the response type as text/plain to avoid content type
confusion.

Issue SPR-14066

Author: Rob Winch

spring-test/src/test/java/org/springframework/test/web/servlet/htmlunit/MockMvcWebClientBuilderTests.java

@@ -139,7 +139,7 @@ public String contextPath(HttpServletRequest request) {
 
 	@RestController
 	static class CookieController {
-		@RequestMapping("/")
+		@RequestMapping(value="/", produces="text/plain")
 		public String cookie(@CookieValue("cookie") String cookie) {
 			return cookie;
 		}

spring-test/src/test/java/org/springframework/test/web/servlet/htmlunit/webdriver/MockMvcHtmlUnitDriverBuilderTests.java

@@ -158,7 +158,7 @@ public String contextPath(HttpServletRequest request) {
 
 	@RestController
 	static class CookieController {
-		@RequestMapping("/")
+		@RequestMapping(value="/", produces="text/plain")
 		public String cookie(@CookieValue("cookie") String cookie) {
 			return cookie;
 		}