[itaku] fix caching bug (#1842)

ItakuApi.user() would always return the first user it was called with,
regardless of its 'username' argument.

gallery_dl/extractor/itaku.py

@@ -163,7 +163,7 @@ class ItakuAPI():
         endpoint = "/galleries/images/{}/".format(image_id)
         return self._call(endpoint)
 
-    @memcache()
+    @memcache(keyarg=1)
     def user(self, username):
         return self._call("/user_profiles/{}/".format(username))