[ArrowWriter] Set schema at first write example
MEMBER
Good point!

I guess we could add this to `write_batch` as well (before using `self._schema` in the first line of this method)?
2020-05-26T23:04:36Z
https://github.com/huggingface/datasets/pull/200#issuecomment-634326004
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2020-05-26T23:04:36Z
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Right now if the schema was not specified when instantiating `ArrowWriter`, then it could be set with the first `write_table` for example (it calls `self._build_writer()` to do so).

I noticed that it was not done if the first example is added via `.write`, so I added it for coherence.