Multi-valued properties of per-document mixins are not preserved when the document is versioned or copied.

Per-instance mixins were implemented by NXP-6084, but they don't work properly yet: values of multi-valued properties of mixins are lost in some cases. To reproduce the loss:

• Add a facet having a schema to a document instance.
• Change the document again and save it, creating a version.

Expected results:

• Both the working copy of the document and the version have the facet and all its values.

Actual results:

• Both the working copy of the document and the version have the facet and all simple values of the schema of the facet, but the version has lost all multivalued properties of the facet (such as "list of strings"). Multi-valued properties are preserved only for the working copy of the document.

The same error can also be reproduced by copying a document which has a mixin with multivalued properties.