See https://caddy.community/t/caddy-server-that-returns-only-ip-address-as-text/6928/6?u=matt
In most cases, we will want to apply header operations immediately,
rather than waiting until the response is written. The exceptions are
generally going to be if we are deleting a header field or if a field is
to be overwritten. We now automatically defer header ops if deleting a
header field, and allow the user to manually enable deferred mode with
the defer subdirective.
Before this change, only response headers could be manipulated with the
Caddyfile's 'header' directive.
Also handle the request Host header specially, since the Go standard
library treats it separately from the other header fields...
Use piles from which to draw config values.
Module values can return their name, so now we can do two-way mapping
from value to name and name to value; whereas before we could only map
name to value. This was problematic with the Caddyfile adapter since
it receives values and needs to know the name to put in the config.
Along with several other changes, such as renaming caddyhttp.ServerRoute
to caddyhttp.Route, exporting some types that were not exported before,
and tweaking the caddytls TLS values to be more consistent.
Notably, we also now disable automatic cert management for names which
already have a cert (manually) loaded into the cache. These names no
longer need to be specified in the "skip_certificates" field of the
automatic HTTPS config, because they will be skipped automatically.
