ParquetSink fails for nullable fields

Before pyarrow 0.15, it is not possible to create pyarrow record batch with schema.

So in apache_beam.io.parquetio._ParquetSink, when creating pyarrow record batch we use 

rb = pa.RecordBatch.from_arrays(arrays, self._schema.names)

Error is raised that the parquet table to be created (record batch schema) has a different schema with the schema specify (self._schema).

For example, when schema specified with "is not null", the record batch schema doesn't indicate that, the error will be raised.

The fix is to use schema instead of names in pa.RecordBatch.from_arrays

rb = pa.RecordBatch.from_arrays(arrays, schema=self._schema)