support marshalling `CHOICE` elements
support marshalling `CHOICE` elements

This change supports marshalling `blpapi::Element` that are of type
`blpapi::DataType::CHOICE` to the appropriate `v8::Value` type.

jmptrader/blpapi-node,apaprocki/blpapi-node,bloomberg/blpapi-node,apaprocki/blpapi-node,bloomberg/blpapi-node,jmptrader/blpapi-node,apaprocki/blpapi-node,bloomberg/blpapi-node,jmptrader/blpapi-node