Update document-api to register our namespaces with ElementTree

[hkthom]

After creating a new Tableau workbook using "Workbook.save_as" of Document API without any changes (Just changing file name ), the content (structure ) of newly generated workbook is different from the original one. There is a difference between the XMLs likely caused by ElementTree modifying the XML namespace keys automatically in the new version of the workbook. This is causing Document API's save_as function to change filter state from "Use all" to "select from list"

This ought to be fixed by modifying the library on github, which is a oneliner

“ET.register_namespace(‘user’, ‘http://tableau.com/user’)

or whatever the url is.