fix(document): avoid stripping out fields in discriminator schema after select: false field

[vkarpov15]

Fix #15308

Summary

When we refactored a forEach() into a for in init(), we left a return that should've become a continue

Examples

[Chee7ah]

I assume this return should become a continue as well then, right?

mongoose/lib/document.js

Lines 747 to 752 in 47b88cd

	for (let index = 0; index < len; ++index) {
	i = keys[index];
	// avoid prototype pollution
	if (i === '__proto__' || i === 'constructor') {
	return;
	}

[hasezoey]

LGTM, i second @Chee7ah suggestion.