fix(misskey-js): content-typeはapplication/jsonでないもののみを記録するように

[kakkokari-gtyih]

What

• content-typeはapplication/jsonでないもののみを記録するように
• content-type が判別できない場合はapplication/jsonをデフォルトにするように

Why

• サイズ削減
• Fix #14502

Additional info (optional)

Checklist

• [x] Read the contribution guide
• [ ] Test working in a local environment
• [ ] (If needed) Add story of storybook
• [ ] (If needed) Update CHANGELOG.md
• [ ] (If possible) Add tests

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 41.57%. Comparing base (3fe7e37) to head (f02f28b). Report is 6 commits behind head on develop.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #14508 +/- ## ============================================ + Coverage 20.17% 41.57% +21.39% ============================================ Files 725 1551 +826 Lines 100502 196640 +96138 Branches 1045 2737 +1692 ============================================ + Hits 20279 81750 +61471 - Misses 79681 114328 +34647 - Partials 542 562 +20 ```

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[syuilo]

🙏🏿