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

proxy server_id_hdr_name member (used for "http-send-name-header" option)
wasn't cleaned up in free_proxy(), resulting in small memory leak if
"http-send-name-header" was used on a regular or default proxy.

This may be backported to all stable versions.

(cherry picked from commit 99f340958211881dc6b48b6dc960d126179c4bcc)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 67d4a9507d6e33db21627ee4552c47c84571b329)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 7ecbf7a5f8f8d0582e89fc2624de2fae3c3dc4d7)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
diff --git a/src/proxy.c b/src/proxy.c
index 71189d6..5ef5409 100644
--- a/src/proxy.c
+++ b/src/proxy.c
@@ -171,6 +171,7 @@
 	free(p->server_state_file_name);
 	free(p->capture_name);
 	istfree(&p->monitor_uri);
+	istfree(&p->server_id_hdr_name);
 	free(p->rdp_cookie_name);
 	free(p->invalid_rep);
 	free(p->invalid_req);