Unused/multiple namespaces will be lost when document is saved

[jacalata]

• If there is no namespace in the document when you begin working with it, none will be added.

• If there is a namespace but it is not used in the document, it will be stripped

• if there are two namespaces (possible but unlikely) one will be saved

  Fix will be something like https://stackoverflow.com/questions/41937624/elementtree-why-are-my-namespace-declarations-stripped-out

Originally posted by @jacalata in https://github.com/tableau/document-api-python/pull/197#discussion_r645193528