Author: Reynold Xin <rxin@databricks.com>

This patch had conflicts when merged, resolved by
Committer: Reynold Xin <rxin@databricks.com>

Closes #6527 from rxin/covariant-equals and squashes the following commits:

e7d7784 [Reynold Xin] [SPARK-7975] Enforce CovariantEqualsChecker