[branch-1.1][SPARK-4355] OnlineSummarizer doesn't merge mean correctly

andrewor14 This backports the bug fix in #3220 . It would be good if we can get it in 1.1.1. But this is minor.

Author: Xiangrui Meng <meng@databricks.com>

Closes #3251 from mengxr/SPARK-4355-1.1 and squashes the following commits:

33886b6 [Xiangrui Meng] Merge remote-tracking branch 'apache/branch-1.1' into SPARK-4355-1.1
91fe1a3 [Xiangrui Meng] fix OnlineSummarizer.merge when other.mean is zero