Revert "SessionRepository.save returns S"

See gh-809

spring-session-core/src/main/java/org/springframework/session/MapSessionRepository.java

@@ -76,9 +76,8 @@ public class MapSessionRepository implements SessionRepository<Session> {
 		this.defaultMaxInactiveInterval = Integer.valueOf(defaultMaxInactiveInterval);
 	}
 
-	public Session save(Session session) {
+	public void save(Session session) {
 		this.sessions.put(session.getId(), new MapSession(session));
-		return session;
 	}
 
 	public Session findById(String id) {

spring-session-core/src/main/java/org/springframework/session/SessionRepository.java

@@ -52,7 +52,7 @@ public interface SessionRepository<S extends Session> {
 	 *
 	 * @param session the {@link Session} to save
 	 */
-	S save(S session);
+	void save(S session);
 
 	/**
 	 * Gets the {@link Session} by the {@link Session#getId()} or null if no