There are times when a request can arrive for a Python function with headers as a list. One of those examples is this header `x-vercel-proxied-for` which apparently is set twice. Example:
`[b'x-vercel-proxied-for', [b'207.81.134.243', b'172.71.147.74']]`
I took a quick scan through the other Python server implementations and I don't think any of them manipulate the value of the HTTP headers, the way the ASGI one does so I think we are good there.
To reproduce:
`curl https://..../ -H "foo: bar" -H "foo: bar"`
Will fail.
Fixes: https://github.com/vercel/vercel/issues/9132
