A Deno.serve handler that takes zero arguments uses the native "no
request" fast path, where the Request object is never materialized for JS.
On that path the HTTP/1.1 server discarded all request headers
(RawRequestHeaders::empty()), but automatic compression is decided by reading
accept-encoding from the stored request headers. With the headers gone,
raw_request_compression always returned Compression::None, so responses
from zero-arg handlers were never compressed — even when the client supported
gzip/brotli and the body was compressible.

This retains just the accept-encoding header on the fast path (when automatic
compression is enabled), so the compression decision works while still avoiding
the full request-header copy / Request materialization. When automatic
compression is disabled, the headers are still skipped entirely as before.

Repro

// zero-arg handler -> native fast path
Deno.serve(() => new Response("compress me ".repeat(200)));

$ curl -s -H 'accept-encoding: gzip, br' -D - localhost:PORT -o /dev/null
# before: no content-encoding
# after:  content-encoding: br

Tests

The existing httpServerAutomaticCompressionSkippedForNoRequestFastPath test
actually encoded the bug (it asserted compression was skipped). It's rewritten
as httpServerAutomaticCompressionAppliesForNoRequestFastPath, asserting
content-encoding: gzip and vary: Accept-Encoding. The automaticCompression: false tests continue to assert no compression.