Fix writing matrices to stdout

[Zinurist]

Small fix for when passing 'ark:-' to the WriteHelper Before it would get a RecursionError, since it doesn't have the attribute _stream. You can test it with this example from the Readme:

import numpy
from kaldiio import WriteHelper
with WriteHelper('ark:-') as writer:
    for i in range(10):
        writer(str(i), numpy.random.randn(10, 10))

[nttcslab-sp-admin]

Thanks!