[SPARK-2210] cast to boolean on boolean value gets turned into NOT((boolean_condition) = 0)

```
explain select cast(cast(key=0 as boolean) as boolean) aaa from src
```
should be
```
[Physical execution plan:]
[Project [(key#10:0 = 0) AS aaa#7]]
[ HiveTableScan [key#10], (MetastoreRelation default, src, None), None]
```

However, it is currently
```
[Physical execution plan:]
[Project [NOT((key#10=0) = 0) AS aaa#7]]
[ HiveTableScan [key#10], (MetastoreRelation default, src, None), None]
```

Author: Reynold Xin <rxin@apache.org>

Closes #1144 from rxin/booleancast and squashes the following commits:

c4e543d [Reynold Xin] [SPARK-2210] boolean cast on boolean value should be removed.