title: Not able to create custom _Id in mongoose
body: <p>I'm trying to updating data with below code but it's not inserting _id for me,
  Also I've used upsert:trure but seems like it's not working too.</p>
  <pre><code>/* UPDATE LiveFeed socket */    
  router.put('/', function(req, res, next) {
      filter=req.params.exchange+req.params.symbol;
      LiveFeed.where({filter}).update({
          _id: filter,
          exchange: req.params.exchange,
          symbol: req.params.symbol,
          timestamp: req.params.timestamp,
          ltp: req.params.ltp,
          close: req.params.close,
          open: req.params.open,
          high: req.params.high,
          low: req.params.low,
          vtt: req.params.vtt,
          atp: req.params.atp,
          oi: req.params.oi,
          spot_price: req.params.spot_price,
          total_buy_qty: req.params.total_buy_qty,
          total_sell_qty: req.params.total_sell_qty,
          lower_circuit: req.params.lower_circuit,
          upper_circuit: req.params.upper_circuit,
          yearly_low: req.params.yearly_low,
          yearly_high: req.params.yearly_high,
          bids: req.params.bids,
          asks: req.params.asks,
          ltt: req.params.ltt
      },
      {
          upsert: true,
          multi: true,
          new: true
      }).exec();
  });
  
  //model
  var mongoose = require('mongoose');
  var LiveFeed = new mongoose.Schema({
      _id: String,
      timestamp: String,
      exchange: String,
      symbol: String,
      ltp: Number,
      close: String,
      open: String,
      high: String,
      low: String,
      vtt: Number,
      atp: Number,
      oi: Number,
      spot_price: Number,
      total_buy_qty: String,
      total_sell_qty: String,
      lower_circuit: Number,
      upper_circuit: Number,
      yearly_low:Number,
      yearly_high: Number,
      bids: String,
      asks: String,
      ltt: String,
      updated_date: {
          type: Date,
          default: Date.now
      },
  },
  {
      _id: false
  });
  module.exports = mongoose.model('LiveFeed', LiveFeed);
  </code></pre>