Mail: added Auth-SSL header to indicate SSL.

Based on a patch by Filipe da Silva.

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 f040ec1..fbcc605 100644
--- a/src/mail/ngx_mail_auth_http_module.c
+++ b/src/mail/ngx_mail_auth_http_module.c
@@ -1173,6 +1173,9 @@
           + sizeof("Auth-SMTP-Helo: ") - 1 + s->smtp_helo.len + sizeof(CRLF) - 1
           + sizeof("Auth-SMTP-From: ") - 1 + s->smtp_from.len + sizeof(CRLF) - 1
           + sizeof("Auth-SMTP-To: ") - 1 + s->smtp_to.len + sizeof(CRLF) - 1
+#if (NGX_MAIL_SSL)
+          + sizeof("Auth-SSL: on" CRLF) - 1
+#endif
           + ahcf->header.len
           + sizeof(CRLF) - 1;
 
@@ -1255,6 +1258,15 @@
 
     }
 
+#if (NGX_MAIL_SSL)
+
+    if (s->connection->ssl) {
+        b->last = ngx_cpymem(b->last, "Auth-SSL: on" CRLF,
+                             sizeof("Auth-SSL: on" CRLF) - 1);
+    }
+
+#endif
+
     if (ahcf->header.len) {
         b->last = ngx_copy(b->last, ahcf->header.data, ahcf->header.len);
     }