[SPARK-19682][SPARKR] Issue warning (or error) when subset method "[[" takes vector index

## What changes were proposed in this pull request?
The `[[` method is supposed to take a single index and return a column. This is different from base R which takes a vector index.  We should check for this and issue warning or error when vector index is supplied (which is very likely given the behavior in base R).

Currently I'm issuing a warning message and just take the first element of the vector index. We could change this to an error it that's better.

## How was this patch tested?
new tests

Author: actuaryzhang <actuaryzhang10@gmail.com>

Closes #17017 from actuaryzhang/sparkRSubsetter.