[SPARK-4355][MLLIB] fix OnlineSummarizer.merge when other.mean is zero

See inline comment about the bug. I also did some code clean-up. @dbtsai I moved `update` to a private method of `MultivariateOnlineSummarizer`. I don't think it will cause performance regression, but it would be great if you have some time to test.
Ah right, I get it now.