SPARK-1822: Some minor cleanup work on SchemaRDD.count()

Minor cleanup following #841.

Author: Reynold Xin <rxin@apache.org>

Closes #868 from rxin/schema-count and squashes the following commits:

5442651 [Reynold Xin] SPARK-1822: Some minor cleanup work on SchemaRDD.count()

(cherry picked from commit d66642e3978a76977414c2fdaedebaad35662667)
Signed-off-by: Reynold Xin <rxin@apache.org>