If-Modified-Since should be unique header line

src/http/ngx_http_request.c

diff --git a/src/http/ngx_http_request.c b/src/http/ngx_http_request.c
index 87b6017..f3d8f48 100644
--- a/src/http/ngx_http_request.c
+++ b/src/http/ngx_http_request.c
@@ -75,7 +75,7 @@
 
     { ngx_string("If-Modified-Since"),
                  offsetof(ngx_http_headers_in_t, if_modified_since),
-                 ngx_http_process_header_line },
+                 ngx_http_process_unique_header_line },
 
     { ngx_string("User-Agent"), offsetof(ngx_http_headers_in_t, user_agent),
                  ngx_http_process_header_line },