[SPARK-12629][SPARKR] Fixes for DataFrame saveAsTable method

I've tried to solve some of the issues mentioned in: https://issues.apache.org/jira/browse/SPARK-12629
Please, let me know what do you think.
Thanks!

Author: Narine Kokhlikyan <narine.kokhlikyan@gmail.com>

Closes #10580 from NarineK/sparkrSavaAsRable.