Fix reddit top image

Article.py set_reddit_top_img never works as self.top_img is not empty string in any case. 
```python
 def set_reddit_top_img(self):
        if self.top_img != u'': # if we already have a top img...
            return
        # select image with top size
```

Reason is simple set_top_img could encode None to "None" string:
```python
  def set_top_img(self, src_url):
        src_url = encodeValue(src_url)
        self.top_img = src_url
        self.top_image = src_url
```