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ARROW-1616: [Python] Add unit test for RecordBatchWriter.write dispatching to write_table or write_batch

I'd actually already implemented this, but it was untested.

Author: Wes McKinney <wes.mckinney@twosigma.com>

Closes #1170 from wesm/ARROW-1616 and squashes the following commits:

32599fe3 [Wes McKinney] Add unit test for _RecordBatchWriter.write