Document BSON type 6 as 'undefined'

E.g.
> db.testcol.insert({v:undefined})
> db.testcol.findOne()
{ "_id" : ObjectId("517ae3259c3c002bf65b97fb"), "v" : null }
> db.testcol.count({v:null})
0
> typeof(db.testcol.findOne().v)
object
> db.testcol.count({v:{$type:6}})
1
> db.testcol.count({v:{$in:[undefined]}})
1

Signed-off-by: kay <kay.kim@10gen.com>