Fix buffer assignment in compression.py

[eslavich]

I missed that we were setting buffer[...] here and not buffer.data[...]! That's the real source of the problem. This is the difference between them:

In [10]: buffer = np.zeros((10,), np.uint8)
    ...: buffer.data[0:5] = b'12345'
    ...: buffer
Out[10]: array([49, 50, 51, 52, 53,  0,  0,  0,  0,  0], dtype=uint8)

In [11]: buffer = np.zeros((10,), np.uint8)
    ...: buffer[0:5] = b'12345'
    ...: buffer
Out[11]: array([57, 57, 57, 57, 57,  0,  0,  0,  0,  0], dtype=uint8)

(where 57 is 12345 % 2**8)

[jdavies-st]

Looks good.