set MIME-type length,
otherwise ngx_http_test_content_type() did not find "image/gif"

src/http/modules/ngx_http_empty_gif_module.c

diff --git a/src/http/modules/ngx_http_empty_gif_module.c b/src/http/modules/ngx_http_empty_gif_module.c
index 8450cae..e6515df 100644
--- a/src/http/modules/ngx_http_empty_gif_module.c
+++ b/src/http/modules/ngx_http_empty_gif_module.c
@@ -122,6 +122,7 @@
         return rc;
     }
 
+    r->headers_out.content_type_len = sizeof("image/gif") - 1;
     r->headers_out.content_type.len = sizeof("image/gif") - 1;
     r->headers_out.content_type.data = (u_char *) "image/gif";