code: public void setExpiration(String expiration) {
          if (expiration == null) {
              _properties.remove(AMQP_PROP_EXPIRATION);
              return;
          }
          
          _properties.put(AMQP_PROP_EXPIRATION, expiration);
      }
docs: Sets the value of "expiration" property.  If a null value is passed
  in, it indicates that the property is not set.
  
  @param  expiration    value of "expiration" property