title: how to set value to Mib using snmp4j
body: <p>i'm trying to create simple example of snmp manager and agent.
  all i need to do is just read the current value from given OID, write some new value to replace it, and read it back.</p>
  
  <p>my get function working well but my set doesn't.
  in the line: <code>PDU responsePDU = event.getResponse();</code>
  responsePDU equal to null and i don't understand why? 
  any help will be very welcome!</p>
  
  <pre><code>    public ResponseEvent set(OID oid,String val) throws IOException 
  {
      PDU pdu = new PDU();
      VariableBinding varBind = new VariableBinding(oid,new OctetString(val));
      pdu.add(varBind);
      pdu.setType(PDU.SET);
      pdu.setRequestID(new Integer32(1));
      Target target=getTargetForWrite();
  
      ResponseEvent event = snmp.set(pdu, target);
      if(event != null)
      {
        System.out.println("\nResponse:\nGot Snmp Set Response from Agent");
        System.out.println("Snmp Set Request = " + event.getRequest().getVariableBindings());
        PDU responsePDU = event.getResponse();
        System.out.println("\nresponsePDU = "+responsePDU);
        if (responsePDU != null)
        {
          int errorStatus = responsePDU.getErrorStatus();
          int errorIndex = responsePDU.getErrorIndex();
          String errorStatusText = responsePDU.getErrorStatusText();
          System.out.println("\nresponsePDU = "+responsePDU);
          if (errorStatus == PDU.noError)
          {
            System.out.println("Snmp Set Response = " + responsePDU.getVariableBindings());
          }
          else
          {
            System.out.println("errorStatus = "+responsePDU);
            System.out.println("Error: Request Failed");
            System.out.println("Error Status = " + errorStatus);
            System.out.println("Error Index = " + errorIndex);
            System.out.println("Error Status Text = " + errorStatusText);
          }
         }
  
        return event;
      }
      throw new RuntimeException("GET timed out");
  }
  
  
  
      private Target getTargetForWrite()
  {
      CommunityTarget target = new CommunityTarget();
      target.setCommunity(new OctetString("private"));
      target.setVersion(SnmpConstants.version2c);
      target.setAddress(GenericAddress.parse(address));
      target.setRetries(2);
      target.setTimeout(1500);
  
      return target;
  }
  </code></pre>