Could not be unmarshalled

Hello,

I tried the jsonix PO example, it works fine with marshalling and unmarshalling.

When I try to implement it in my use case I am getting the following error
`Error: Element [{http://www.w3.org/2001/XMLSchema}xsd:schema] could not be unmarshalled as is not known in this context and the property does not allow DOM content.`

Steps:

1. created INVOICE.js in mappings folder using below command
`java -jar \lib\jsonix-schema-compiler-full-2.3.9.jar -d mappings -p INVOICE https://docs.oasis-open.org/ubl/os-UBL-2.1/xsd/maindoc/UBL-Invoice-2.1.xsd`

2. stored the same xsd (https://docs.oasis-open.org/ubl/os-UBL-2.1/xsd/maindoc/UBL-Invoice-2.1.xsd) as an example in invoice.xml
```
const Jsonix = require('jsonix').Jsonix,
INVOICE = require('../mappings/INVOICE'),
context = new Jsonix.Context([INVOICE]),
unmarshaller = context.createUnmarshaller();

unmarshaller.unmarshalFile('../mappings/invoice.xml',
function (unmarshalled) {
console.log(unmarshalled.value.cac:InvoicePeriod.cbc:StartDate);
});
```

Any help will be highly appreciated. Thanks

PS: code [jsonixInvoice](https://github.com/dishantindept/jsonixInvoice) for reference

Start with mappings/INVOICE.js and mappings/invoice.xml, then reproduce the failure through context.createUnmarshaller() and unmarshalFile(). Compare this case with the working JSONix PO example and the jsonixInvoice reference repository. Done when the supplied invoice.xml unmarshals without the unknown-element error and the callback can access the invoice data.

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