lib/Element/Uri.php
public function xmlSerialize(Xml\Writer $writer)
    {
        $writer->text(
            \Sabre\Uri\resolve(
                $writer->contextUri,
                $this->value
            )
        );
    }
public function xmlSerialize(Xml\Writer $writer)
    {
        $writer->text(
            \Sabre\Uri\resolve(
                $writer->contextUri,
                $this->value
            )
        );
    }
The xmlSerialize metod is called during xml writing.

Use the $writer argument to write its own xml serialization.

An important note: do _not_ create a parent element. Any element
implementing XmlSerializble should only ever write what's considered
its 'inner xml'.

The parent of the current element is responsible for writing a
containing element.

This allows serializers to be re-used for different element names.

If you are opening new elements, you must also close them again.