Align WebSession#save implementations with API

Closes gh-1135

Author: vpavic

spring-session-data-redis/src/integration-test/java/org/springframework/session/data/redis/ReactiveRedisOperationsSessionRepositoryITests.java

@@ -30,6 +30,7 @@
 import org.springframework.test.context.web.WebAppConfiguration;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
 
 /**
  * Integration tests for {@link ReactiveRedisOperationsSessionRepository}.
@@ -209,7 +210,10 @@ public void changeSessionSaveOldSessionInstance() {
 		this.repository.save(session).block();
 
 		toSave.setLastAccessedTime(Instant.now());
-		this.repository.save(toSave).block();
+
+		assertThatExceptionOfType(IllegalStateException.class)
+				.isThrownBy(() -> this.repository.save(toSave).block())
+				.withMessage("Session was invalidated");
 
 		assertThat(this.repository.findById(sessionId).block()).isNull();
 		assertThat(this.repository.findById(session.getId()).block()).isNotNull();

spring-session-data-redis/src/main/java/org/springframework/session/data/redis/ReactiveRedisOperationsSessionRepository.java

@@ -156,7 +156,9 @@ public Mono<Void> save(RedisSession session) {
 					session.hasChangedSessionId() ? session.originalSessionId
 							: session.getId());
 			return this.sessionRedisOperations.hasKey(sessionKey)
-					.flatMap((exists) -> exists ? result : Mono.empty());
+					.flatMap((exists) -> exists ? result
+							: Mono.error(new IllegalStateException(
+									"Session was invalidated")));
 		}
 	}