RedisIO emit error of EXEC without MULTI

RedisIO has EXEC without MULTI error after SET a batch of records.

By looking at the source code, I guess there is missing `pipeline.multi();` after exec() the last batch.

https://github.com/apache/beam/blob/master/sdks/java/io/redis/src/main/java/org/apache/beam/sdk/io/redis/RedisIO.java#L555