[SPARK-1959] String "NULL" shouldn't be interpreted as null value

JIRA issue: [SPARK-1959](https://issues.apache.org/jira/browse/SPARK-1959)

Author: Cheng Lian <lian.cs.zju@gmail.com>

Closes #909 from liancheng/spark-1959 and squashes the following commits:

306659c [Cheng Lian] [SPARK-1959] String "NULL" shouldn't be interpreted as null value