Properly set host header to ascii string in ProxyFixMiddleware.

[apollo13]

While playing with supporting trusted_cidrs in ProxyFixMiddleware I noticed that mypy/pyright cannot narrow the type properly when:

if scope["type"] in {"http", "websocket"}:

is used.

After rewriting that to:

if scope["type"] == "http" or scope["type"] == "websocket":

I was able to remove the type: ignore statements and discovered another bug, namely that the host header is set as a string and not as a bytes object.

I think the type narrowing is quite useful here, so maybe we want to keep it? An alternative would be to write it like this:

if scope["type"] == "lifespan":
   pass
else:
   ... existing code

though that does not end well if another scope type is added in the future.

No strong feeling if you want to keep the in {} check instead and ignore the typing, but the host header value should be bytes either way :D

[apollo13]

Oh and type narrowing for a set with literals will probably not get supported ever: microsoft/pyright#6871 (comment)