fix(populate): allow deselecting discriminator key when populating

[vkarpov15]

Fix #3230
Re: #13760
Re: #13679

Summary

Right now, populating with __t: 0 in projection throws a "Cannot do inclusion on field prop in exclusion projection", whereas deselecting projection works fine outside of populate(). The issue looks to be an ordering issue: Mongoose isn't correctly handling __t: 0 as the first property in the select object, and with populate() Mongoose was reversing key order, so { name: 1, __t: 0 } was ending up as { __t: 0, name: 1 } because of utils.object.shallowCopy.

Both issues are fixed: Mongoose now handles __t: 0 as first property in select object (utils.object.shallowCopy() was removed, no longer necessary with spread operator), and populate() with { __t: 0 } now works as expected 👍

Examples