Check for None when setting admin in user_modify

api method. Fixes a bug where setting an admin-enabled user back to a normal
user is not possible (issue #100).

synadm/api.py

@@ -550,7 +550,7 @@ def user_modify(self, user_id, password, display_name, threepid,
             ]})
         if avatar_url:
             data.update({"avatar_url": avatar_url})
-        if admin:
+        if admin is not None:
             data.update({"admin": admin})
         if deactivation == "deactivate":
             data.update({"deactivated": True})