`format_pretty_xml()` indents badly in a few cases

[nat-goodspeed]

>>> print(llsd.format_pretty_xml({'ha': ['a', 'b', 'c']}).decode())
<?xml version="1.0" ?>
<llsd>
<map>
    <key>ha</key>
    <array>
      <string>a</string>
      <string>b</string>
      <string>c</string>
</array>
  </map>
</llsd>

This affects autobuild.xml files rewritten by autobuild.