## What changes were proposed in this pull request?

This PR proposes to replace the deprecated `json(RDD[String])` usage to `json(Dataset[String])`.

This currently produces so many warnings.

## How was this patch tested?

Fixed tests.

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #17071 from HyukjinKwon/SPARK-15615-followup.