Authorize.net: fix for the "check if the user already submitted this transaction"

This is not working - because earlier on $authorizeNetFields['x_invoice_num'] has been truncated to 20 digits - so it will never be the same as the the invoice_id stored in CiviCRM:

  // Authorize.Net will not refuse duplicates, so we should check if the user already submitted this transaction                                                                                      
    if ($this->checkDupe($authorizeNetFields['x_invoice_num'], CRM_Utils_Array::value('contributionID', $params))) {
      return self::error(9004, 'It appears that this transaction is a duplicate...');
    }

Turns out - Authorize.net is perfectly happy receiving invoice_id > 20 digits - it will truncate them itself. iATS Payments used to truncate them - now they are showing full sequence for invoice_id.

Bottom line: lets send the entire non truncated invoice_id over to Authorize.net:

• it will be there in full if/when authorize.net decides to retain all digits
• it will give us a checkDupe that will work

PR coming for both 4.7 and 4.6