Headers are not parsed correctly in ASGI middleware #983

The ASGI middleware seems to get the raw headers from the scope which are all in byte form. This is used to get/start a transaction, which does not see the sentry-trace header as it checks for the string version of that header.

See this line for the faulty part:
https://github.com/getsentry/sentry-python/blob/master/sentry_sdk/integrations/asgi.py#L133

There is a method that parses the headers and turns it into a Dict[str, str] found here:
https://github.com/getsentry/sentry-python/blob/master/sentry_sdk/integrations/asgi.py#L228