If I use the `Marshaller` interface on a type to output a slice, it fails with the following error :

`WriteArray can only write a Array while positioned on a Element or Value but is positioned on a TopLevel`

You can see this in https://play.golang.org/p/Sc5Fk_ACvpB

From what I investigated I understood that the call to `bson.Marshal` I do inside `MarshalBSON` thinks the slice is at top level and so refuse to marshal it. Is there an other way to marshal bson w/o that constraint ?