ManualCallback build -> cancel

[kevinkunstmann]

Mpay Error Message: BIF: incorrect parameter.
If the value of cancel is (bool) false, it will never be appended.
The element is required, when the payment should be done, in case of Payment type Paypal Express.
Before:

        if ($this->cancel) {
            $cancel = $this->document->createElement('cancel', $this->cancel);
            $paymentCallback->appendChild($cancel);
        }

After:

$cancel = $this->document->createElement('cancel', $this->cancel);
            $paymentCallback->appendChild($cancel);