Update generator.context with the reordered list

For templates not using pagination, they will fetch directly the articles from the context.

Problem description:
The context was updated already in `generate_context` with:

    self._update_context(('articles', 'dates', 'tags', 'categories',
                              'tag_cloud', 'authors', 'related_posts'))

This plugin is called afterwards:

    signals.article_generator_finalized.send(self)

In the plugin the article list is updated, but not the context. When a template is called the context contains the original article list, not the updated one. If the template fetch the articles from the context then the plugin seems to have no effect.

This patch corrects it. Not sure if asigning the article list directly to the context can cause some secondary effect.