CDRIVER-457 allow dbref's in bson_validate

when bson_validate is provided BSON_VALIDATE_DOLLAR_KEYS, correctly
handle the dbref style sub doc.

I.e. validate:

{
    sub_doc : {
        $ref  : "string",
        $id   : "anything",
        $db   : "optional string",
        extra : "stuff"
    }
}