tq_exchange does not pull dual-listed stocks

[genaromv]

Dual-listed stocks, such as Verizon (VZ), do not appear to be pulled using tq_exchange('NYSE'). I did some sleuthing and discovered they appeared to be stored in the following API call:

https://api.nasdaq.com/api/screener/stocks?tableonly=true&exchange=dual

This is instead of using 'nyse' for the API call:

https://api.nasdaq.com/api/screener/stocks?tableonly=true&exchange=nyse

It would be good if tq_exchange() either integrated these stocks in their proper exchanges or allowed the use of a 'DUAL' parameter value to account for these stocks.