4.0.0-rc2: Setter not called on array of refs

[brandom]

I have a depopulate plugin so that I can call .set on a document with a populated object and avoid casting errors.

depopulate: function(schema, options) {
  schema.eachPath(function(path, type) {
    if (path !== '_id' && type.options.type === Schema.Types.ObjectId || type.caster && type.caster.instance === 'ObjectID') {
      schema.path(path).set(function(val) {
        if (_.isArray(val) && val[0] && val[0]._id) {
          return _.pluck(val, '_id');
        }
        return (_.isObject(val) && val._id) ? val._id : val || undefined;
      });
    }
  });
}

This code works as expected on 3.8.x but fails for arrays of refs on 4.0.0-rc2 resulting in a caster validation error before the setter is reached.

I believe I have traced the issue to this commit: 9ba0fb5 which applies caster setters for each object in an array before performing the setter on the array itself. I am not familiar enough with the code to know whether or not this was intentional, however it does break the above plugin.

I fixed this issue by removing the early return and placing the array caster setters after the path setters. I will submit a PR, but this may need input from the devs/original commiter regarding the implications of changing the order of operations here.