'Changing the tags on a document removes the old tags from
that document.'

def test_new_revision_POST_removes_old_tags(self):

    self.d.current_revision = None
    self.d.save()
    tags = [u'tag1', u'tag2', u'tag3']
    self.d.tags.add(*tags)
    result_tags = list(self.d.tags.names())
    result_tags.sort()
    eq_(tags, result_tags)
    tags = [u'tag1', u'tag4']
    data = new_document_data(tags)
    data['form-type'] = 'rev'
    self.client.post(reverse('wiki.edit', args=[self.d.slug]), data)
    result_tags = list(self.d.tags.names())
    result_tags.sort()
    eq_(tags, result_tags)
