[Minor] Utilise {rcpt_mailer} macro in milter mode

Issue: #2151

src/libserver/milter.c

@@ -1349,6 +1349,11 @@ rspamd_milter_macro_http (struct rspamd_milter_session *session,
 				found->begin, found->len);
 	}
 
+	IF_MACRO("{rcpt_mailer}") {
+		rspamd_http_message_add_header_len (msg, MAILER_HEADER,
+				found->begin, found->len);
+	}
+
 	if (milter_ctx->client_ca_name) {
 		IF_MACRO ("{cert_issuer}") {
 			rspamd_http_message_add_header_len (msg, CERT_ISSUER_HEADER,

src/libserver/protocol.c

@@ -497,7 +497,7 @@ rspamd_protocol_handle_headers (struct rspamd_task *task,
 				}
 				break;
 			default:
-				msg_debug_protocol ("unknown header: %V", hn);
+				msg_debug_protocol ("generic header: %V", hn);
 				break;
 			}
 

src/libserver/protocol_internal.h

@@ -82,5 +82,6 @@
 #define MILTER_HEADER "Milter"
 #define FILENAME_HEADER "Filename"
 #define CERT_ISSUER_HEADER "TLS-Cert-Issuer"
+#define MAILER_HEADER "Mailer"
 
 #endif //RSPAMD_PROTOCOL_INTERNAL_H