encode XML as bytes in render_footnotes()

[jhpyle]

I got an error from docxcompose.composer.add_footnotes() after upgrading docxtpl to 0.19.0.

ValueError: Unicode strings with encoding declaration are not supported. Please use bytes input or XML fragments without declaration.

I was able to avoid this error by making the change in this pull request, which ensures that the modified XML is written back to the docx document as bytes.

I'm not an authority on whether part._blob should be bytes or not, but it seems that way based on how this change fixes the problem I was having with docxcompose.