set_array_compression function appears to not work

[drdavella]

Consider the following example code:

import asdf
import numpy as np

# Create separate arrays so that they use separate data blocks
zlib_data = np.array([x for x in range(1000)])
bzp2_data = np.array([x for x in range(1000)])

tree = dict(zlib_data=zlib_data, bzp2_data=bzp2_data)
with asdf.AsdfFile(tree) as af:
    af.set_array_compression(zlib_data, 'zlib')
    af.set_array_compression(bzp2_data, 'bzp2')
    af.write_to(outfile)

In this case, neither block ends up being compressed. Presumably, the compression algorithm ends up being overridden by the default value of the all_array_compression parameter to write_to.