Image filter: downgrade strong etags to weak ones as needed.

src/http/modules/ngx_http_image_filter_module.c

diff --git a/src/http/modules/ngx_http_image_filter_module.c b/src/http/modules/ngx_http_image_filter_module.c
index c983b97..bd7a309 100644
--- a/src/http/modules/ngx_http_image_filter_module.c
+++ b/src/http/modules/ngx_http_image_filter_module.c
@@ -1012,6 +1012,7 @@
     b->last_buf = 1;
 
     ngx_http_image_length(r, b);
+    ngx_http_weak_etag(r);
 
     return b;
 }