Able to update documents to have $ref $id and $db fields on top level

> db.test.drop()

true

> db.test.update({_id:0}, {_id:0, $ref: "1", $id: 1, $db:"1"}, true)

WriteResult({ "nMatched" : 0, "nUpserted" : 1, "nModified" : 0, "_id" : 0 })

> db.test.findOne()

{ "_id" : 0, "$ref" : "1", "$id" : 1, "$db" : "1" }

> db.test.insert({_id: 1, $ref: "1", $id: 1, $db: "1"})

WriteResult({

        "nInserted" : 0,

        "writeError" : {

                "code" : 2,

                "errmsg" : "Document can't have $ prefixed field names: $ref"

        }

})

> db.test.find()

{ "_id" : 0, "$ref" : "1", "$id" : 1, "$db" : "1" }

> db.runCommand({buildinfo:1})

{

        "version" : "2.7.2-pre-",

        "gitVersion" : "87d09bc9cfe898f093c4d76ca5aad807c2e817e7",

        "OpenSSLVersion" : "",

        "sysInfo" : "Linux localhost.localdomain 3.13.10-200.fc20.x86_64 #1 SMP Mon Apr 14 20:34:16 UTC 2014 x86_64 BOOST_LIB_VERSION=1_49",

        "loaderFlags" : "-fPIC -pthread -Wl,-z,now -rdynamic",

        "compilerFlags" : "-Wnon-virtual-dtor -Woverloaded-virtual -std=c++11 -fPIC -fno-strict-aliasing -ggdb -pthread -Wall -Wsign-compare -Wno-unknown-pragmas -Winvalid-pch -pipe -Werror -O3 -Wno-unused-local-typedefs -Wno-unused-function -Wno-deprecated-declarations -fno-builtin-memcmp -std=c99",

        "allocator" : "tcmalloc",

        "versionArray" : [

                2,

                7,

                2,

                -100

        ],

        "javascriptEngine" : "V8",

        "bits" : 64,

        "debug" : false,

        "maxBsonObjectSize" : 16777216,

        "ok" : 1

}

>