DomDriver UTF-8 not working

I am using XStream 1.0.2. The following two codes produce the same output:

--------------- Sample 1 --------------
final XStream x1 = new XStream(new DomDriver("UTF-8"));
x1.toXML(teststring, new FileWriter("test-encoding-1.xml"));

--------------- Sample 2 --------------
final XStream x2 = new XStream(new DomDriver());
x2.toXML(teststring, new FileWriter("test-encoding-2.xml"));

--------------- End Samples -----------

Given the teststring variable is "Ää Öö Üu", the output for both is identical:

00000000h: 3C 73 74 72 69 6E 67 3E ; <string>
00000008h: C4 E4 20 D6 F6 20 DC FC ; Ää Öö Üü
00000010h: 3C 2F 73 74 72 69 6E 67 ; </string
00000018h: 3E ; >

Obviously, the umlaut encoding does not take place in UTF-8, which (to my expectations) would look like this:

00000000h: 3C 73 74 72 69 6E 67 3E ; <string>
00000008h: C3 84 C3 A4 20 C3 96 C3 ; Ää Ã-Ã
00000010h: B6 20 C3 9C C3 BC 3C 2F ; ¶ Üü</
00000018h: 73 74 72 69 6E 67 3E ; string>

The encoding of the above code snippets rather take place in my Windows XPs default encoding schema ISO-8859-1.