[8.x] Fix loadAggregate not correctly applying casts

[aalyusuf]

Currently "loadExists" only applies the "bool cast" on the first model in a collection and skips the rest.

[driesvints]

If you can, a test that proves the bug fix could be helpful here.

[taylorotwell]

I am closing this pull request because it lacks sufficient explanation, tests, or both. It is difficult for us to merge pull requests without these things because the change may introduce breaking changes to the framework.

Feel free to re-submit your change with a thorough explanation of the feature and tests - integration tests are preferred over unit tests. Please include it's benefit to end users; the reasons it does not break any existing features; how it makes building web applications easier, etc.

Thanks!