Don't pass `oldtext` kwarg to mwclient `save()` method
    
    As a convenience, wikitcms allows you to pass an `oldtext` kwarg
    to its `Page.save()` method and will skip the remote roundtrip
    if the `oldtext` and the text you asked it to save are the same.
    But if you use this convenience mechanism (as relval does) and
    the text is *not* the same, wikitcms will pass the `oldtext`
    kwarg along to the parent `save()` method (from mwclient), which
    does not understand it and will print a warning about it. So
    let's stop doing that, to avoid the warning.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>