[SPARK-8908] [SQL] Add () to distinct definition in dataframe

Adding `()` to the definition of `distinct` in DataFrame allows distinct to be called with parentheses, which is consistent with `dropDuplicates`.

Author: Cheolsoo Park <cheolsoop@netflix.com>

Closes #7298 from piaozhexiu/SPARK-8908 and squashes the following commits:

7f0d923 [Cheolsoo Park] Add () to distinct definition in dataframe