fix: query fails for subclassed Parse object in Parse relation

[fischerscode]

New Pull Request Checklist

• I am not disclosing a vulnerability.
• I am creating this PR in reference to an issue.

Issue Description

Related issue: #696

Approach

Due to the deletion of the development branch #697 was closed automatically.
I've decided to cherry pick 4829c36 and (re-)open the PR to avoid loosing a contribution.

This change is not tested by me. Since I didn't use relations yet.

CC: @kutear

TODOs before merging

• Add tests
• Add changes to documentation (guides, repository pages, in-code descriptions)
• A changelog entry

[parse-github-assistant]

Thanks for opening this pull request!

• 🎉 We are excited about your hands-on contribution!

[codecov]

Codecov Report

Merging #763 (a8f66b9) into master (d80ca58) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master     #763   +/-   ##
=======================================
  Coverage   10.28%   10.28%           
=======================================
  Files          48       48           
  Lines        2810     2810           
=======================================
  Hits          289      289           
  Misses       2521     2521           

Impacted Files	Coverage Δ
packages/dart/lib/src/objects/parse_relation.dart	0.00% <0.00%> (ø)

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[RodrigoSMarques]

@fischerscode Your PR has not been lost. Here

[fischerscode]

Closing this, since it is included in #762.