Fix bug not remove child private field

[taina0407]

Fixing bug in case that private field in child field.

For example:

const thisSchema = new Schema({
  parent: {
    child: {
      type: [String],
      private: true,
    }
  },
}, options);

When calling .toJSON(), child still appear because current removePrivatePaths trying to access ret['parent.child'] instead of ret.parent.child

[adamreisnz]

You need to make child it's own schema, then you can use private: true correctly.

[taina0407]

While mongoose still allow user to create child documents without schema, I think the plugin should also support it.
Or at least mention this on README.
Thanks

[adamreisnz]

Can you make a fix for it that doesn't introduce another dependency?

[adamreisnz]

Looks like you have made changes to your own fork of this package and you are merging in unnecessary changes with this PR. You can clean that up if you want and leave only the fixes in, otherwise I'll close it without merging.