Getters are double called when using get() into a document array

[vkarpov15]

Popped up when investigating #6637

const mongoose = require('mongoose');
mongoose.connect('mongodb://localhost:27017/gh6637', { useNewUrlParser: true });
const conn = mongoose.connection;
const Schema = mongoose.Schema;

mongoose.set('debug', true);

const schema = new Schema({
  nested: {
    arr: [{ key: String }]
  }
});

schema.path('nested.arr.0.key').get(v => 'foobar' + v);

const Test = mongoose.model('test', schema);

const test = new Test;

test.nested.arr.push({ key: 'value' });
test.nested.arr.push({ key: 'value2' });

async function run() {
  await conn.dropDatabase();
  await test.save();
  let doc = await Test.findOne();
  console.log(doc.get('nested.arr.0.key')); // "foobarfoobarvalue"
}

run();