backout r3315 and test r->header_only last, since it's not actually
frequent here: 304 and HEAD responses are not set it before the filter

src/http/modules/ngx_http_gzip_filter_module.c

diff --git a/src/http/modules/ngx_http_gzip_filter_module.c b/src/http/modules/ngx_http_gzip_filter_module.c
index 19835b6..879971f 100644
--- a/src/http/modules/ngx_http_gzip_filter_module.c
+++ b/src/http/modules/ngx_http_gzip_filter_module.c
@@ -243,7 +243,6 @@
     conf = ngx_http_get_module_loc_conf(r, ngx_http_gzip_filter_module);
 
     if (!conf->enable
-        || r->header_only
         || (r->headers_out.status != NGX_HTTP_OK
             && r->headers_out.status != NGX_HTTP_FORBIDDEN
             && r->headers_out.status != NGX_HTTP_NOT_FOUND)
@@ -251,7 +250,8 @@
             && r->headers_out.content_encoding->value.len)
         || (r->headers_out.content_length_n != -1
             && r->headers_out.content_length_n < conf->min_length)
-        || ngx_http_test_content_type(r, &conf->types) == NULL)
+        || ngx_http_test_content_type(r, &conf->types) == NULL
+        || r->header_only)
     {
         return ngx_http_next_header_filter(r);
     }