fix(populate): handle specifying recursive populate as a string with discriminators

[vkarpov15]

Summary

A bug that I ran into earlier today. If you populate() on a discriminator model and set the populate option to a string as follows:

return Log.find({}).
              populate({
                path: 'activity',
                populate: 'postedBy'
              }).
              sort({ seq: -1 });

You'll get the following error:

TypeError: Cannot create property 'strictPopulate' on string 'postedBy'

This fixes that case

Examples

[AbdelrahmanHafez]

LGTM, thanks 👍