Update to Boot 2.0.0.M4

Issue: gh-4608

gradle.properties

@@ -1,3 +1,3 @@
 gaeVersion=1.9.56
-springBootVersion=2.0.0.M2
+springBootVersion=2.0.0.M4
 version=5.0.0.BUILD-SNAPSHOT

samples/boot/oauth2login/src/main/java/sample/web/MainController.java

@@ -53,7 +53,9 @@ public class MainController {
 			.getProviderDetails().getUserInfoEndpoint().getUri();
 		if (!StringUtils.isEmpty(userInfoEndpointUri)) {	// userInfoEndpointUri is optional for OIDC Clients
 			userAttributes = this.webClient
+				.mutate()
 				.filter(oauth2Credentials(authentication))
+				.build()
 				.get()
 				.uri(userInfoEndpointUri)
 				.retrieve()