BUG/MINOR: proxy: fix header_unique_id leak on deinit()

proxy header_unique_id wasn't cleaned up in proxy_free_defaults(),
resulting in small memory leak if "unique-id-header" was used on a
default proxy section.

It may be backported to all stable versions.

(cherry picked from commit 847c406b9a040193c374daad3269d646dda7dbcc)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 310c0dd14ea23f9d027ac86c20af071d2a6127ac)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 4b215f2db95fb2d5ec369d409dc29dce0d72769f)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
diff --git a/src/proxy.c b/src/proxy.c
index 768d963..0034810 100644
--- a/src/proxy.c
+++ b/src/proxy.c
@@ -1498,6 +1498,7 @@
 #if defined(CONFIG_HAP_TRANSPARENT)
 	ha_free(&defproxy->conn_src.bind_hdr_name);
 #endif
+	istfree(&defproxy->header_unique_id);
 	istfree(&defproxy->server_id_hdr_name);
 
 	http_ext_clean(defproxy);