Add a handler to every route adding response header

[peb-adr]

... if $HSTS is set at startup

[peb-adr]

I'm a bit unsure how good this solution really is.
It seems a bit cumbersome to iterate over each route to add N handlers that will add the response headers, but I couldn't find a more central place to take care of it in caddy's docs.

[jsangmeister]

Sounds sensible