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

从 examples/fastapi/async_app.py 以及 app_handler.handle(request) 使用的 FastAPI AsyncSlackRequestHandler 路径开始。使用包含“ö”的消息复现 mention listener，并在跟踪 raw body 处理的同时将其与 ASCII 消息进行比较。当包含特殊字符的消息不再返回 403，同时正常的请求处理仍然有效时，即表示完成。