Fix malformed JSON in _buildQueryRelational

[juriSacchetta]

Pull Request

• Report security issues confidentially.
• Any contribution is under this license.
• Link this pull request to an issue.

Issue

https://github.com/parse-community/Parse-SDK-Flutter/issues/939 Closes: #939

Approach

Fix bug in JSON creation when whereMatchesQuery is used.

Tasks

• [ ] Add tests
• [ ] Add changes to documentation (guides, repository pages, code comments)

[parse-github-assistant[bot]]

Thanks for opening this pull request!

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.01 :tada:

Comparison is base (bae8902) 39.50% compared to head (4326e04) 39.52%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #943 +/- ## ========================================== + Coverage 39.50% 39.52% +0.01% ========================================== Files 60 60 Lines 3339 3340 +1 ========================================== + Hits 1319 1320 +1 Misses 2020 2020 ``` | [Impacted Files](https://app.codecov.io/gh/parse-community/Parse-SDK-Flutter/pull/943?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=parse-community) | Coverage Δ | | |---|---|---| | [packages/dart/lib/src/network/parse\_query.dart](https://app.codecov.io/gh/parse-community/Parse-SDK-Flutter/pull/943?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=parse-community#diff-cGFja2FnZXMvZGFydC9saWIvc3JjL25ldHdvcmsvcGFyc2VfcXVlcnkuZGFydA==) | `45.28% <100.00%> (+0.20%)` | :arrow_up: |

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[mbfakourii]

It seems that the tests failed!

It is possible to check the tests