When creating an unversioned page and switching to versioned and back the wrong parent is used

Steps to reproduce:

• create an unversioned page inside a book
• switch to versioned using the 'Versions' menu
• switch back to unversioned

Expected behavior:

• The document is copied to the WebHome of the same space

Actual behavior:

• A new sibling page is crated with the name of the book

Reason:
When creating a Page as unversioned initially page document has a parent property set to the name of the book. When creating it as versioned first the document does not have parent set at all.
The current way of getting the parent varies on whether that property is present or not.