use "localhost" in "Host" header line, if unix socket is used in "auth_http"

src/mail/ngx_mail_auth_http_module.c

diff --git a/src/mail/ngx_mail_auth_http_module.c b/src/mail/ngx_mail_auth_http_module.c
index c755c72..b9f9635 100644
--- a/src/mail/ngx_mail_auth_http_module.c
+++ b/src/mail/ngx_mail_auth_http_module.c
@@ -1405,7 +1405,14 @@
 
     ahcf->peer = u.addrs;
 
-    ahcf->host_header = u.host;
+    if (u.family != AF_UNIX) {
+        ahcf->host_header = u.host;
+
+    } else {
+        ahcf->host_header.len = sizeof("localhost") - 1;
+        ahcf->host_header.data = (u_char *) "localhost";
+    }
+
     ahcf->uri = u.uri;
 
     if (ahcf->uri.len == 0) {