[gfycat] consistent 'userName' values for 'user' downloads (#1962)

by using the name from the input URL and not relying on possibly faulty
or incomplete API results.

'userData[username]', if available, will still have the original name.

gallery_dl/extractor/gfycat.py

@@ -73,6 +73,9 @@ class GfycatUserExtractor(GfycatExtractor):
         "count": ">= 100",
     })
 
+    def metadata(self):
+        return {"userName": self.key}
+
     def gfycats(self):
         return GfycatAPI(self).user(self.key)