[SPARK-4841] fix zip with textFile()

UTF8Deserializer can not be used in BatchedSerializer, so always use PickleSerializer() when change batchSize in zip().

Also, if two RDD have the same batch size already, they did not need re-serialize any more.
Can we reproduce the bug by creating an RDD of string directly? It is simpler than touching disk. It is also helpful to put the JIRA number in the comment.