fixed JIRA:CSHARP-42 related to linq and an empty field selection list.  Since it was a derived type, we were adding the discriminator which caused MongoDB to only bring back the discriminator.
fixed JIRA:CSHARP-42 related to linq and an empty field selection list.  Since it was a derived type, we were adding the discriminator which caused MongoDB to only bring back the discriminator.

zh-huan/mongodb,mongodb-csharp/mongodb-csharp,samus/mongodb-csharp