[SPARK-1913][SQL] Bug fix: column pruning error in Parquet support

JIRA issue: [SPARK-1913](https://issues.apache.org/jira/browse/SPARK-1913)

When scanning Parquet tables, attributes referenced only in predicates that are pushed down are not passed to the `ParquetTableScan` operator and causes exception.

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

Closes #863 from liancheng/spark-1913 and squashes the following commits:

f976b73 [Cheng Lian] Addessed the readability issue commented by @rxin
f5b257d [Cheng Lian] Added back comments deleted by mistake
ae60ab3 [Cheng Lian] [SPARK-1913] Attributes referenced only in predicates pushed down should remain in ParquetTableScan operator