Fix reddit top image

[otemnov]

Article.py set_reddit_top_img never works as self.top_img is not empty string in any case.

 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:

  def set_top_img(self, src_url):
        src_url = encodeValue(src_url)
        self.top_img = src_url
        self.top_image = src_url

[codelucas]

You are right, thank you for finding this subtle bug! Before merging it, don't you think a better fix would be having our check be if self.top_img: return versus if self.top_img != u'': return?

I think we should just change the clause to be if top_img: instead of if self.top_img != u''. Or, maybe at least change the default return value in https://github.com/codelucas/newspaper/blob/master/newspaper/parsers.py#L98 to be u"" instead of None?

Feel free to send in another PR and i'll most likely merge. I don't know if making such a big change in the encode method is such a good idea.

[codelucas]

I changed my mind, I like your fix more. Merging now.