ModbusRTUClient: restore default RS485 object

Completes arduino-libraries@1c51a13 and fixes arduino-libraries#64

Author: facchinm

src/ModbusRTUClient.h

@@ -41,7 +41,7 @@ class ModbusRTUClientClass : public ModbusClient {
   int begin(RS485Class& rs485, unsigned long baudrate, uint16_t config = SERIAL_8N1);
 
 private:
-  RS485Class* _rs485;
+  RS485Class* _rs485 = &RS485;
 };
 
 extern ModbusRTUClientClass ModbusRTUClient;