use return value in | if it's already a Stream

Author: MichaelKim0407

stream/__init__.py

@@ -18,7 +18,11 @@ def __or__(self, other: _Function[_T_co, _V_co]) -> 'Stream[_V_co]':
         Override the `|` operator.
         :param other: An iterator function, see `functional` package.
         """
-        return IterStream(other(self))
+        result = other(self)
+        if isinstance(result, Stream):
+            return result
+        else:
+            return IterStream(result)
 
     def __gt__(self, other: _Call[_T_co, _V]) -> _V:
         """