parseOrderBook() should use existing "nonce"
parseOrderBook() should use existing "nonce"

There are Exchange implementations that set the "nonce" value to the orderbook object passed to the parseOrderBook method, but this gets discarded, being huobipro one of such exchanges. Other implementations set the "nonce" value after calling this method.

The proposed change allows for implementations like huobipro's to properly work, where they directly return the result of parseOrderBook()
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