ParquetWriter.Dispose() can now be called multiple times

[SartorialOffense]

ParquetWriter.Dispose() can now be called multiple times without degenerate writes to the underlying stream.

Fixes

Issue # Multiple calls to ParquetWriter.Dispose() cause degenerate writes to the underlying stream.

Description

It should be possible to call Dispose() multiple times on an object without consequence. The current implementation of ParquetWriter does not track its disposal state and will rewrite data to the stream every time Dispose() is called.

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