Revert "cmd/derper: stop setting content header in handleBootstrapDNS"

Didn't help enough. We are setting another header anyway. Restore it.

This reverts commit 60abeb027b.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f