Fixes jpfuentes2#366.

Fixes jpfuentes2#364.

Before this change, some eager loaded nested relationships were wiped out
when some first-level relationships were the same model object.

For example, when 2 `Post`s have the same `Author` and you load posts
with option `'include' => ['author' => ['profile']]`, then first loaded
post will have eager loaded author with eager loaded profile, while
the second post will have eager loaded author (cloned), but without
eager loaded profile. `clone` method just doesn't clone the relationships
of cloned author.

This leads to a serious performance degradation. Because you expect all
relationships are eagerly loaded when traverse them in the code, while
in reality only some of them are loaded, and others generate many
unnecessary SQL queries to load them on-the-fly.

After this change, nested relationships will be cloned recursively
instead of wiping them out completely. This leads to expected behavior,
when all eagerly loaded relationships will be preserved and fixes the
performance degradation.