Keep the leading delimiter in read_block with split_before and no length

[LuisFigueroaG]

With length=None, read_block returns before applying split_before, so the block loses its leading delimiter, unlike a read with an explicit length that reaches the end of the file:

>>> f = io.BytesIO(b"123\n456\n789")
>>> read_block(f, 1, None, b"\n", split_before=True)
b'456\n789'
>>> read_block(f, 1, 100, b"\n", split_before=True)
b'\n456\n789'

Now both return b'\n456\n789'. The new test fails on master.