[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.
Sorry if this a dumb question -- and this isn't a change in this PR -- but why can't a sample of value 0 be added to the summarizer?