ARROW-6714: [R] Fix untested RecordBatchWriter case

[nealrichardson]

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.

[codecov-io]

Codecov Report

Merging #5518 into master will decrease coverage by 11.41%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           master    #5518       +/-   ##
===========================================
- Coverage   88.69%   77.28%   -11.42%     
===========================================
  Files         964       58      -906     
  Lines      128215     4010   -124205     
  Branches     1501        0     -1501     
===========================================
- Hits       113724     3099   -110625     
+ Misses      14126      911    -13215     
+ Partials      365        0      -365

Impacted Files	Coverage Δ
r/R/record-batch-writer.R	88.88% <100%> (+28.88%)	⬆️
python/pyarrow/ipc.pxi
cpp/src/parquet/column_page.h
cpp/src/plasma/test/external_store_tests.cc
cpp/src/arrow/array/builder_decimal.cc
cpp/src/plasma/client.cc
cpp/src/arrow/flight/internal.cc
cpp/src/arrow/compute/compute_test.cc
cpp/src/arrow/python/io.cc
python/pyarrow/hdfs.py
... and 898 more

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