Need to be able to unmarshall to bson.D field within a struct

One of the data types in the tools resembles this:

noformat
type foo struct

noformat

Even with the Stitch-based patches to mgo/bson, this fails unmarshaling with the error "reflect.MakeSlice of non-slice type".

(N.B. Changing the Bar field to *bson.M works.)

This ticket is a reminder that after the planned changes to the BSON encoder and mgobson, we need this particular use case to work to help users who want to use mgobson as a stepping stone converting to the Go driver.