[SPARK-15908][R] Add varargs-type dropDuplicates() function in SparkR

## What changes were proposed in this pull request?

This PR adds varargs-type `dropDuplicates` function to SparkR for API parity.
Refer to https://issues.apache.org/jira/browse/SPARK-15807, too.

## How was this patch tested?

Pass the Jenkins tests with new testcases.

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #13684 from dongjoon-hyun/SPARK-15908.