fix(multipart): don't share MultipartParseOptions._DEFAULT_HANDLERS

[myusko]

Summary of Changes

Fix a problem, when MultipartParseOptions._DEFAULT_HANDLERS was shared between different instances

Related Issues

Fixes #2293.

Pull Request Checklist

• [x] Applied changes to both WSGI and ASGI code paths and interfaces (where applicable).
• [x] Added tests for changed code.
• [x] Prefixed code comments with GitHub nick and an appropriate prefix.
• [x] Coding style is consistent with the rest of the framework.
• [] Updated documentation for changed code.
  • [x] Added docstrings for any new classes, functions, or modules.
  • [x] Updated docstrings for any modifications to existing code.
  • [x] Updated both WSGI and ASGI docs (where applicable).
  • [x] Added references to new classes, functions, or modules to the relevant RST file under docs/.
  • [x] Updated all relevant supporting documentation files under docs/.
  • [x] A copyright notice is included at the top of any new modules (using your own name or the name of your organization).
  • [x] Changed/added classes/methods/functions have appropriate versionadded, versionchanged, or deprecated directives.
• [x] Changes (and possible deprecations) have towncrier news fragments under docs/_newsfragments/, with the file name format {issue_number}.{fragment_type}.rst. (Run towncrier --draft to ensure it renders correctly.)

[codecov[bot]]

Codecov Report

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

Project coverage is 100.00%. Comparing base (1f914c5) to head (16c8d5b). Report is 1 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2322 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 63 63 Lines 7499 7502 +3 Branches 1275 1275 ========================================= + Hits 7499 7502 +3 ```

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