Allow virtual nested objects to work

[rdougan]

I noticed that nested virtuals are supported, but do not work if they are an object:

const schema = new mongoose.Schema({
  nested: {
    test: {
      a: String
    }
  }
}, { id: false });

schema.virtual('nested.test2').get(function() {
  return this.nested.test;
});

Does not work:

.lean({virtuals: ['nested.test2']});

I've fixed it, added a test, and verified all existing tests pass.