Support casting JSON form of buffers

[vkarpov15]

The below currently throws cast error:

const mongoose = require('mongoose');

mongoose.set('debug', true);

run();

async function run() {
  await mongoose.connect('mongodb://localhost:27017/test');

  const Model = mongoose.model('Foo', new mongoose.Schema({
    buf: Buffer
  }));
  
  const doc = await Model.create({
    buf: {
      type: 'Buffer',
      data: [ 72, 101, 108, 108, 111, 44, 32, 87, 111, 114, 108, 100 ] 
    }
  });
}

That's bad because the output JSON.stringify({ buf: Buffer.from('test') }); is '{"buf":{"type":"Buffer","data":[116,101,115,116]}}'

[Fonger]

In node.js, Buffer.from({ type: 'Buffer', data: [1, 2, 3] }) is valid too.
It makes sense to cast this to Buffer automatically. I've submitted a pull request for this feature.