[mod_auth] preserve WWW-Authenticate for error docs (fixes #2730)
[mod_auth] preserve WWW-Authenticate for error docs (fixes #2730)

fixes auth broken by 9973fdb0:
  [core] reset response headers, write_queue for error docs

x-ref:
  "http auth does not send WWW-Authenticate header in git rev 598cdd0"
  https://redmine.lighttpd.net/issues/2730

lighttpd/lighttpd1.4,PatchyFog/lighttpd1.4-mbedtls,lighttpd/lighttpd1.4,PatchyFog/lighttpd1.4-mbedtls,PatchyFog/lighttpd1.4-mbedtls,PatchyFog/lighttpd1.4-mbedtls,gstrauss/lighttpd1.4,gstrauss/lighttpd1.4,lighttpd/lighttpd1.4,PatchyFog/lighttpd1.4-mbedtls,lighttpd/lighttpd1.4,lighttpd/lighttpd1.4,gstrauss/lighttpd1.4,gstrauss/lighttpd1.4,PatchyFog/lighttpd1.4-mbedtls,lighttpd/lighttpd1.4,PatchyFog/lighttpd1.4-mbedtls,gstrauss/lighttpd1.4,gstrauss/lighttpd1.4