Spring StringDecoder does not provide a way to configure the default charset

i want to write a chatclient for a very old chat system. that chat system uses iso-8859-1 encoding. when i use the webclient, german umlaute (äü etc.) are messed up and there is no way to somehow configure that in StringDecoder. when i tell the webclient to use accept headers with utf8, thats simply ignored as the server can not respond in utf8.

there are 2 or 3 static constructors to allow me to create a StringDecoder but all use utf8!

public static StringDecoder textPlainOnly(List<String> delimiters, boolean stripDelimiter) {
    return new StringDecoder(delimiters, stripDelimiter, new MimeType("text", "plain", DEFAULT_CHARSET));
}

and DEFAULT_CHARSET is UTF-8.

Provide a way to configure that, thanks!