[deviantart] update usernames (#455)

In the case that a user changed his username, requesting deviations
with an old name might cause problems (missing deviations, etc.)

The internal 'username' value therefore now gets updated to the
current username taken from the user profile.

gallery_dl/extractor/deviantart.py

@@ -58,9 +58,12 @@ def skip(self, num):
 
     def items(self):
         if self.user:
-            self.group = not self.api.user_profile(self.user)
+            profile = self.api.user_profile(self.user)
+            self.group = not profile
             if self.group:
                 self.subcategory = "group-" + self.subcategory
+            else:
+                self.user = profile["user"]["username"]
 
         yield Message.Version, 1
         for deviation in self.deviations():