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ARROW-6714: [R] Fix untested RecordBatchWriter case

I noticed this now-defunct call to `table()` while reviewing another PR. We clearly weren't testing this case because if you were to pass a data.frame in, you'd get a segfault. This patch adds tests and fixes the issue.

Closes #5518 from nealrichardson/record-batch-writer-fix and squashes the following commits:

afad9fef5 <Neal Richardson> Fix untested RecordBatchWriter case

Authored-by: Neal Richardson <neal.p.richardson@gmail.com>
Signed-off-by: Neal Richardson <neal.p.richardson@gmail.com>