[SPARK-8686] [SQL] DataFrame should support `where` with expression represented by String

DataFrame supports `filter` function with two types of argument, `Column` and `String`. But `where` doesn't.

Author: Kousuke Saruta <sarutak@oss.nttdata.co.jp>

Closes #7063 from sarutak/SPARK-8686 and squashes the following commits:

180f9a4 [Kousuke Saruta] Added test
d61aec4 [Kousuke Saruta] Add "where" method with String argument to DataFrame