Fix data from previous elements overwriting optional fields

Example:
<book>
  <author>One</author>
  <name>The first book</name>
</book>
<book>
  <name>Two</name>
</book>

The author of the second book will also be "One"
Implementation: Support attributes
Test attributes

. Various variants of writing them
. Adjust expected outcome in international_use() test
Add test verifying @[attribute] support in CreationOf()