Use the backend server charset

The default character set should be copied from the server so that
MaxScale appears to be the same. This fixes problems where utf8mb4
couldn't be taken into use because MaxScale would always send latin1 as
the server charset.

server/modules/protocol/mysql_client.c

@@ -277,6 +277,11 @@ int MySQLSendHandshake(DCB* dcb)
     int len_version_string = 0;
     int id_num;
 
+    if (dcb->service->dbref)
+    {
+        mysql_server_language = dcb->service->dbref->server->charset;
+    }
+
     MySQLProtocol *protocol = DCB_PROTOCOL(dcb, MySQLProtocol);
     GWBUF *buf;