Fix document not applying manual populate when using a function in schema.options.ref

[IchirokuXVI]

Summary

Schema.options.ref works with functions but it doesn't mark the path as populated if done manually when creating a new document or assigning to a property.

Examples

const mongoose = require("mongoose");

mongoose.connect("mongodb://localhost:27017");

const test = async () => {
  const userSchema = new mongoose.Schema({
    username: { type: String },
    phone: { type: mongoose.Schema.Types.ObjectId, ref: "phones" },
  });

  const userSchemaWithFunc = new mongoose.Schema({
    username: { type: String },
    phone: {
      type: mongoose.Schema.Types.ObjectId,
      ref: function () {
        return "phones";
      },
    },
  });

  const phoneSchema = new mongoose.Schema({
    phoneNumber: { type: String },
    extension: { type: String },
  });

  const User = mongoose.model("users", userSchema);
  const UserWithFunc = mongoose.model("usersWithFunc", userSchemaWithFunc);
  const Phone = mongoose.model("phones", phoneSchema);

  const phone = await Phone.create({
    phoneNumber: "123456789",
    extension: "123",
  });

  const user = await User.create({
    username: "John Doe",
    phone,
  });

  const userWithFunc = await UserWithFunc.create({
    username: "John Doe",
    phone,
  });

  console.log(user.populated("phone")); // Returns ObjectId
  console.log(userWithFunc.populated("phone")); // Returns undefined
};

test()
  .then(() => console.log("Finished"))
  .catch((error) => console.error(error))
  .finally(() => process.exit(0));

[vkarpov15]

I made a couple of minor changes to make this more consistent with ref function checks in other places, but otherwise LGTM. I'll add a test after merging.