Correctly clean up nested subdocs modified state on save()

[vkarpov15]

Summary

Fix #13609. This issue was pretty tricky at first glance, and I spent a lot of time trying to figure out how to make it so that pull() would mark paths in default state as modified. But then I realized the simple solution was to just always call $__reset() on every subdoc 😄 Given that $__reset() is no longer a bottleneck, this is a better solution because we can now consistently ensure that every individual subdocs' state is reset on save().

Examples