NEW YORK (Reuters) - In March, noted short-seller Andrew Left took a swipe at Netflix Inc's stock, calling the online video company's aggressive spending on content acquisition "unsustainable."The stock took a brief dive on the Citron Research chief's assertion, but the volume of bets against it has since dropped to the lowest in at least half a decade.Netflix bonds are a different story.Bearish bets against the company's $8.4 billion of junk-rated bonds have more than tripled this year to an..