How do I control REXML formatting on added elements?

I am a adding a couple of simple elements to a bunch of XML files (plists). The existing XML element I am working on looks like this:

<dict>
  <key>background</key>
  <string>#FFFFFF</string>
  <key>caret</key>
  <string>#000000</string>
  <key>foreground</key>
  <string>#000000</string>
  <key>invisibles</key>
  <string>#BFBFBF</string>
  <key>lineHighlight</key>
  <string>#00000012</string>
  <key>selection</key>
  <string>#BAD6FD</string>
</dict>

I have captured this element in an object called settings and I am adding new <key> and <string> elements, and adding some text to those elements. Simple enough so far:

settings.add_element('key').add_text('gutter')
settings.add_element('string').add_text('#282828')

Trouble is, when I write this back out, the XML looks like this (note the last line):

<dict>
  <key>background</key>
  <string>#FFFFFF</string>
  <key>caret</key>
  <string>#000000</string>
  <key>foreground</key>
  <string>#000000</string>
  <key>invisibles</key>
  <string>#BFBFBF</string>
  <key>lineHighlight</key>
  <string>#00000012</string>
  <key>selection</key>
  <string>#BAD6FD</string>
<key>gutter</key><string>#282828</string></dict>

I am using the write (REXML::Document) method to write out the XML (to $stdout at the moment):

tmtheme.write( $stdout )

Also tried

tmtheme.write( $stdout, 2 )

But these don't return the desired results. The following looked promising:

tmtheme.write( $stdout, 2, true )

But this gives me a known error. Update: just tried it on Ruby 1.9 and although I don't get the erro, it doesn't help - I still get the formatting as seen in the example above.

Can anyone tell me how I can format the XML so that it conforms to the formatting style of the rest of the document? It doesn't necessarily need to be done with REXML.

Solution

Robert Klemme and Aaron Patterson came up with the goods on the ruby-talk list.

This is the first time a question I've posed on SoF has drawn a complete blank.