[SPARK-19066][SPARKR][BACKPORT-2.1] LDA doesn't set optimizer correctly

## What changes were proposed in this pull request?
Back port the fix to SPARK-19066 to 2.1 branch.

## How was this patch tested?
Unit tests

Author: wm624@hotmail.com <wm624@hotmail.com>

Closes #16623 from wangmiao1981/bugport.