`write_to` silently fails to write `asdf_library`

[braingram]

import asdf
af = asdf.AsdfFile()
af["asdf_library"] = 1
af.write_to("foo.asdf")

Passes with no warnings or errors. However foo.asdf contains:

asdf_library: !core/software-1.0.0 {author: The ASDF Developers, homepage: 'http://github.com/asdf-format/asdf',
  name: asdf, version: 3.0.2.dev317+gdbbc26f5}