text: Title: Polygon all_tickers returns empty list on Weekends
  Body: Running the following gives an empty list of `Symbol` objects when run on weekends.
  
  ```
  import alpaca_trade_api as tradeapi
  
  api = tradeapi.REST()
  tickers = api.polygon.all_tickers()
  tickers
  []
  ```
  
  Checked the implementation and looks like it grabs the most recent day snapshot which may not work on a Saturday (which is when us retail guys are doing some research). Took a look and a kind of invasive approach could be to swap this out for a call to grouped daily, but not sure. 