Setting core_properties.last_modified_by produces an invalid document in some cases.

Problem

After setting last_modified_by and saving, Word reports the file as corrupted. This may be due to XML namespace issues or element ordering in the core properties part.

Investigation needed

• Reproduce the issue with a minimal test case
• Compare the XML output with a valid document
• Check if the cp:lastModifiedBy element is being created with correct namespace

Upstream: python-openxml#1037 (17 comments)