How can I set a specific (Content-Type) HTTP response header by configuring undertow?

Summary

I'm implementing a web service, that is deployed on a Wildfly-15 application server. I'd like to have the Content-Type HTTP response header to include charset=UTF-8. This is necessary for the client to understand my response.

Details

One of my clients always sends its request without specifying charset in Content-Type. Wildfly's web service stack in this case uses the default charset=ISO-8859-1 in the response. See this behavior in undertow code, lines 602-611:

private String extractCharset(HeaderMap headers) {
    String contentType = headers.getFirst(Headers.CONTENT_TYPE);
    if (contentType != null) {
        String value = Headers.extractQuotedValueFromHeader(contentType, "charset");
        if (value != null) {
            return value;
        }
    }
    return ISO_8859_1;
}

Because my response is, in fact, UTF-8-encoded (and its need to be), this causes trouble on the client side. See the header dumps in the undertow log:

----------------------------REQUEST---------------------------
        header=Connection=Keep-Alive
        header=SOAPAction=****            
        header=Accept-Encoding=gzip,deflate
        header=Content-Type=text/xml
        header=Content-Length=2137
        header=User-Agent=Apache-HttpClient/4.1.1 (java 1.5)
        header=Host=****
        locale=[]
        method=POST
        ....
 --------------------------RESPONSE--------------------------
        header=Connection=keep-alive
        header=Content-Type=text/xml;charset=ISO-8859-1
        header=Content-Length=1553
        header=Date=Tue, 29 Jan 2019 16:19:38 GMT
        status=200

This kind of response leads to the following exception in an IBM Websphere application:

org.springframework.ws.InvalidXmlException: Could not parse XML; nested exception is org.xml.sax.SAXParseException: An invalid XML character (Unicode: 0xffffffff) was found in the element content of the document.

Unfortunately, modifying the client is not an option.

Unsuccessful experiments

My efforts so far went into trying to configure Widlfy's undertow via specifying filters to override HTTP Response headers. I was able to set Content-Encoding only, with the help of Response Header filter. It seems, the Content-Type header is overridden somewhere else along the line.

My second guess was to use Expression Filter with the expression

header(header=Content-Type, value='text/xml;charset=UTF-8')

Unfortunately, it didn't work also.

Question

• Can this problem be solved by the configuration of undertow?
• Or should I somehow (how?) programmatically set the HTTP response header to include charset=UTF-8?