Clean up trailers in ngx_http_clean_header() as well.

The function has not been updated with introduction of trailers support
in 7034:1b068a4e82d8 (1.13.2).

src/http/ngx_http_special_response.c

diff --git a/src/http/ngx_http_special_response.c b/src/http/ngx_http_special_response.c
index 76e6705..72f56fd 100644
--- a/src/http/ngx_http_special_response.c
+++ b/src/http/ngx_http_special_response.c
@@ -575,6 +575,10 @@
     r->headers_out.headers.part.next = NULL;
     r->headers_out.headers.last = &r->headers_out.headers.part;
 
+    r->headers_out.trailers.part.nelts = 0;
+    r->headers_out.trailers.part.next = NULL;
+    r->headers_out.trailers.last = &r->headers_out.trailers.part;
+
     r->headers_out.content_length_n = -1;
     r->headers_out.last_modified_time = -1;
 }