Fix include_optional_linkage_data with joins

[pjungwir]

When the user adds a join in records, it confuses get_join_arel_node because it finds the same number of joins before & after adding a join for include_optional_linkage_data (or equivalently for always_include_*_linkage_data).

This commit falls back to searching the existing arel nodes for a compatible join and uses that if found.

Fixes #1449

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• Maintains compliance with JSON:API
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[averydev]

This has had a hugely positive impact on performance for us. Thank you!

[lgebhardt]

This looks good to me. Thanks @pjungwir