[SNMP4J] Minor optimization in PDU.toString() method

[Matthieu Casanova]

Hi, il the last release 1.6b (but probably in previous one) there is a minor
optimization to do :

The method contains line 601 this
buf.append(getErrorStatusText()+"("+errorStatus+")");

Since we have a StringBuffer it would be better to use it for those String
concatenations like that
buf.append(getErrorStatusText());
buf.append('(');
buf.append(errorStatus);
buf.append(')');

Matthieu