Stream not closing after terminal operation

[garlontas]

Describe the bug
Streams are returning results, although they have been consumed.

To Reproduce

stream = Stream.of([1, 2, 3, 4, 5, 6])
print(stream.sum()) # 21
print(stream.filter(lambda x: x % 2 == 0).reduce(lambda x, y: x + y)) # Optional(12)

Expected behavior
Line 3 should fail with error: Stream has been closed

[garlontas]

Closed by #42