The description.NewServer function currently takes a result.IsMaster, but that adds an extra step of decoding indirection. Instead, take the code from operation.Result and use it to update the description.NewServer method.
The description.NewServer function is only being used in the SDAM spec tests and operation.IsMaster.Result duplicates the code to create a new server description. We should change this so IsMaster.Result uses this function and remove the duplication.