Ensure hassio does not do blocking I/O to get forwarded host

[bdraco]

Proposed change

request.host can fallback to doing blocking I/O because of aio-libs/aiohttp#9308

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[home-assistant]

Hey there @home-assistant/supervisor, mind taking a look at this pull request as it has been labeled with an integration (hassio) you are listed as a code owner for? Thanks!

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[bdraco]

Should we raise here if forward_host is None?

Can that ever happen

[bdraco]

I think this can never happen for hassio so its not something to worry about

[bdraco]

I think we would have to forward http/1.0 with no host set for this to happen...so not never but very unlikely

If some addon messes it up it could cause core to do blocking I/O so probably worth protecting against