Fix polymorphic eager load with foreign_key as String.

The foreign_key could be `String` and just doing `owners_map[owner_key]`
could return `nil`.
To prevent this bug, we should `to_s` both keys if their types are
different.

Fixes #14734.
Merge pull request #14855 from laurocaetano/fix_polymorphic_with_string_key

Fix polymorphic eager load with foreign_key as String.