403 when using special characters

Issue: When using special characters like "ö" app_handler.handle(request) will return Unauthorized.

Tools: I'm using FastAPI AsyncSlackRequestHandler adapter. I don't know if this happends in without adapters on others, this is my use case.

With this code can be reproduced: https://github.com/slackapi/bolt-python/blob/main/examples/fastapi/async_app.py

Simply using the mention listener and as a user on slack send a special character, 403 will be returned.

Hypothesis: I believe is the way the raw body is handled, that when using special characters is not well decoded or something, I'll keep looking to see if I find a solution, in the meantime reporting this as issue since seems pretty clear is one

examples/fastapi/async_app.py と、app_handler.handle(request) で使用される FastAPI AsyncSlackRequestHandler のパスから始めます。 “ö” を含むメッセージで mention listener を再現し、raw body の処理を追跡しながら ASCII メッセージと比較します。特殊文字を含むメッセージが 403 を返さなくなり、通常のリクエスト処理が引き続き機能すれば完了です。