fix(pivot): only use unique classes in the pivot union (Fixes #1606)

[pataar]

Summary

This makes sure that we won't add a class that's already present in the union type for pivots. This can happen when multiple relations use the same Pivot class.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update
• Misc. change (internal, infrastructure, maintenance, etc.)

Checklist

• Existing tests have been adapted and/or new tests have been added
• Add a CHANGELOG.md entry
• Update the README.md
• Code style has been fixed via composer fix-style

[pataar]

@barryvdh I saw that you're planning to remove the CHANGELOG.md. I ignored it in this PR to avoid any conflicts.