This is a workaround for some proxy setup, where the ETag header gets
stripped from the response headers unless there is a Content-Type header
set.

In particular, we saw this bug when putting Cloudflare in front of
Synapse.
I'm pretty sure this is a Cloudflare bug, as this behaviour isn't
documented anywhere, and doesn't make sense whatsoever.

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Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>