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"
        }
    }
Consider adding a test for when $db is present but is not the third field, similar to the ```$ref with $id not first keys``` test you have above.