How to save black and white images

[NewPy]

The following code save the numpy array as red/blue strips, how can I save
an array as grey scale image?

import numpy as np
import mahotas
img = np.ones([10,10])
img[::2,:]=0
mahotas.imsave('test.png', img)


Another example, read an image in as grey scale, imsave produce color
image, how to preserve it as grey scale?
s_file='Nuts.png'
img = mahotas.imread(s_file, as_grey=True)
mahotas.imsave('test.png', img)

Thanks