Fix include_optional_linkage_data with joins (#1450)

[aytigra]

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.

All Submissions:

• I've checked to ensure there aren't other open Pull Requests for the same update/change.
• I've submitted a ticket for my issue if one did not already exist.
• My submission passes all tests. (Please run the full test suite locally to cut down on noise from travis failures.)
• I've used Github auto-closing keywords in the commit message or the description.
• I've added/updated tests for this change.

New Feature Submissions:

• I've submitted an issue that describes this feature, and received the go ahead from the maintainers.
• My submission includes new tests.
• My submission maintains compliance with JSON:API.

Bug fixes and Changes to Core Features:

• I've included an explanation of what the changes do and why I'd like you to include them.
• I've provided test(s) that fails without the change.

Test Plan:

Reviewer Checklist:

• Maintains compliance with JSON:API
• Adequate test coverage exists to prevent regressions