Fix bug with decoding timestamp as decimal

[Jefffrey]

Closes #14

[Jefffrey]

Thanks @progval for identifying this bug

[Jefffrey]

Bug was here. Secondary is unsigned stream. It's retrieved correctly for timestamp, but was mistakenly taken as signed here. See line 62 for correct behaviour.

orc-rust/src/array_decoder/timestamp.rs

Lines 53 to 72 in f934c8e

	fn get_inner_timestamp_decoder<T: ArrowTimestampType + Send>(
	column: &Column,
	stripe: &Stripe,
	seconds_since_unix_epoch: i64,
	) -> Result<PrimitiveArrayDecoder<T>> {
	let data = stripe.stream_map().get(column, Kind::Data);
	let data = get_rle_reader(column, data)?;
	let secondary = stripe.stream_map().get(column, Kind::Secondary);
	let secondary = get_unsigned_rle_reader(column, secondary);
	let present = PresentDecoder::from_stripe(stripe, column);
	let iter = Box::new(TimestampDecoder::<T>::new(
	seconds_since_unix_epoch,
	data,
	secondary,
	));
	Ok(PrimitiveArrayDecoder::<T>::new(iter, present))
	}