ZO-4267: mock connector fix, should check for existence in content di…

…ct and not in the locks dict

core/src/zeit/connector/mock.py

@@ -245,7 +245,7 @@ def unlock(self, id, locktoken=None):
         return locktoken
 
     def locked(self, id):
-        if id not in self._locked:
+        if id not in self._data:
             raise KeyError(id)
         id = self._get_cannonical_id(id)
         return self._locked.get(id, (None, None, False))