Simplify call to AttributeValue::getValue when UserInfo::getAttribute() is called with 2 parameters

If `func_num_args()` is 2 then we can avoid `call_user_func_array` and call `$av->getValue()` directly ([that's faster](http://stackoverflow.com/questions/8343399/calling-a-function-with-explicit-parameters-vs-call-user-func-array))