[SPARK-9182] [SQL] Filters are not passed through to jdbc source

This PR fixes unable to push filter down to JDBC source caused by `Cast` during pattern matching.

While we are comparing columns of different type, there's a big chance we need a cast on the column, therefore not match the pattern directly on Attribute and would fail to push down.

Author: Yijie Shen <henry.yijieshen@gmail.com>

Closes #8049 from yjshen/jdbc_pushdown.