Fix invalid tax if zero

[yogi70]

on a tax-less invoice the generated XML did not contain some values where they are expected:

<ubl:Invoice …>
  …
  <cac:TaxTotal>
    <cac:TaxSubtotal>
      <cbc:TaxableAmount>226.60</cbc:TaxableAmount>
      <cbc:TaxAmount>0.00</cbc:TaxAmount>
      <cac:TaxCategory>
        <cbc:ID>K</cbc:ID>
        <cbc:Percent>0.00</cbc:Percent>
        <cbc:TaxExemptionReasonCode>VATEX-EU-IC</cbc:TaxExemptionReasonCode>
        <cac:TaxScheme>
          <cbc:ID>VAT</cbc:ID>
        </cac:TaxScheme>
      </cac:TaxCategory>
    </cac:TaxSubtotal>
  </cac:TaxTotal>
  …
</ubl:Invioce>

it validates if I manually add some data:
ubl:Invoice.cac:TaxTotal.cbc:TaxAmount and twice the currencyID Attribute to ubl:Invoice.cac:TaxTotal.cac:TaxSubtotal.{cbc:TaxableAmount,cbc:TaxAmount}

<ubl:Invoice …>
  …
  <cac:TaxTotal>
    <cbc:TaxAmount currencyID="EUR">0.00</cbc:TaxAmount>
    <cac:TaxSubtotal>
      <cbc:TaxableAmount currencyID="EUR">226.60</cbc:TaxableAmount>
      <cbc:TaxAmount currencyID="EUR">0.00</cbc:TaxAmount>
      <cac:TaxCategory>
        <cbc:ID>K</cbc:ID>
        <cbc:Percent>0.00</cbc:Percent>
        <cbc:TaxExemptionReasonCode>VATEX-EU-IC</cbc:TaxExemptionReasonCode>
        <cac:TaxScheme>
          <cbc:ID>VAT</cbc:ID>
        </cac:TaxScheme>
      </cac:TaxCategory>
    </cac:TaxSubtotal>
  </cac:TaxTotal>
  …
</ubl:Invioce>

Also fixed the forwarded parameteres on XmlDocumentReader:

• .whenNotExists()
  the reference could only be given to $callbackElse, not the the $callback function
• .wnehNotEquals()
  added the reference values to the $callbackElse as on .whenEquals()

[horstoeko]

Hi @yogi70,

looks good. Can you add your test to ./build/phpunit.xml - otherwise the test will not run... ;)

[yogi70]

Added the test to the build/phpunit.xml file