PropertiesConverter should sort by key

The PropertiesConverter [1] currently serializes its map of key-value entries without ordering them first. Thus the resulting XML may change between runs, which is not very diff-friendly.

A possible solution could look like this (in PropertiesConverter.marshal(...)):

TreeSet s = new TreeSet(properties.keySet());

for (Iterator iterator = s.iterator(); iterator.hasNext() {
Object key = iterator.next();
Object value = properties.get(key);

Cheers,
Christopher

[1] - http://xstream.codehaus.org/javadoc/com/thoughtworks/xstream/converters/collections/PropertiesConverter.html