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

Inizia con examples/fastapi/async_app.py e con il percorso FastAPI AsyncSlackRequestHandler usato da app_handler.handle(request). Riproduci il listener delle menzioni con un messaggio contenente “ö” e confrontalo con un messaggio ASCII, tracciando al contempo la gestione del raw body. Il lavoro è completato quando i messaggi con caratteri speciali non restituiscono più 403, mentre la normale gestione delle richieste continua a funzionare.