Don't append all authors to WXR if `author` arg provided

[roborourke]

Feature Request

• Yes, I reviewed the contribution guidelines.

Describe your use case and the problem you are facing

When exporting content with the author arg set the export contains all the authors still. My use case is that I would like to export data per user in order to send that export over to them - I'm decommissioning a site and want to provide people the opportunity to reuse that information.

I want to do this without leaking the personal data of other users.

Describe the solution you'd like

If the author arg is passed to the command don't prepend all authors to each WXR file. There only needs to be a single author.