Fix non-callback query command calls

`Cursor.prototype.close` was changed to execute the command without a callback parameter like this (9e4b0ceb2b800dfcb56ddeb5cc8cce69ff045c3d):

     this.db._executeQueryCommand(command, {read:self.read, raw:self.raw, connection:self.connection});

`Db.prototype._executeQueryCommand` assumes this 2 argument call to be `command, callback` and unpacks parameters according to that and looses the options object in process. I changed it to ensure that second parameter is actually a function.

I had continuous logging of `[conn822108] killcursors: found 0 of 1` messages on mongodb.log (related: LearnBoost/mongoose#1700) and this fix resolves the issue.