[SPARK-17902][R] Revive stringsAsFactors option for collect() in SparkR

## What changes were proposed in this pull request?

This PR proposes to revive `stringsAsFactors` option in collect API, which was mistakenly removed in https://github.com/apache/spark/commit/71a138cd0e0a14e8426f97877e3b52a562bbd02c.

Simply, it casts `charactor` to `factor` if it meets the condition, `stringsAsFactors && is.character(vec)` in primitive type conversion.

## How was this patch tested?

Unit test in `R/pkg/tests/fulltests/test_sparkSQL.R`.

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #19551 from HyukjinKwon/SPARK-17902.