Default use WebSessionServerSecurityContextRepository

Issue: gh-4719

config/src/main/java/org/springframework/security/config/web/server/ServerHttpSecurity.java

@@ -98,7 +98,7 @@ public class ServerHttpSecurity {
 
 	private ReactiveAuthenticationManager authenticationManager;
 
-	private ServerSecurityContextRepository serverSecurityContextRepository;
+	private ServerSecurityContextRepository serverSecurityContextRepository = new WebSessionServerSecurityContextRepository();
 
 	private ServerAuthenticationEntryPoint serverAuthenticationEntryPoint;