[twitter] always provide an 'author' field (#831, #833)

The idea was to have less metadata clutter for most Tweets were
'author' and 'user' are the same (non-retweets), and only provide
a 'user' field.

The original Tweet author could be gotten with
{author[…]|user[…]}, but basically no one knows about that.

gallery_dl/extractor/twitter.py

@@ -154,6 +154,8 @@ class TwitterExtractor(Extractor):
 
         if "author" in tweet:
             tdata["author"] = self._transform_user(tweet["author"])
+        else:
+            tdata["author"] = tdata["user"]
 
         return tdata