Added support for multiple vwd keys

[hjortlund]

In the real_time_price class method, the price_payload query now gets adjusted - depending on the type of vwdIdentifierType is returned. This is needed for some products e.g. ES (E-mini S&P 500) to work

[george-cpp]

I have had this issue even with normal stocks (Apple), I tested hjrotlund's modification and it works.