How do I properly add a 'Z' at the end of a datetime that's marshalled by Spring MVC with JAXB?

I have a DateTimeXmlAdapter that I'm using to convert Joda dates to Strings. It looks as follows:

public class DateTimeXmlAdapter extends XmlAdapter<String, DateTime> {
    private static final String PATTERN = "yyyy-MM-dd'T'HH:mm:ssZ";
    private static final DateTimeFormatter formatter = DateTimeFormat.forPattern(PATTERN);

    @Override
    public DateTime unmarshal(String value) {
        return formatter.parseDateTime(value);
    }

    @Override
    public String marshal(DateTime value) {
        return formatter.print(value);
    }
}

This results in the following string in XML:

2014-10-16T18:31:57-0400

However, the endpoint I'm sending this to expects a 'Z' at the end instead of -0400. They recommend converting to UTC so timezone information doesn't need to be sent. Makes sense to me. However, I can't seem to get a 'Z' to be at the end unless I state it literally. As in:

private static final String PATTERN = "yyyy-MM-dd'T'HH:mm:ss'Z'";

The discussion below seems to indicate that adding the Z is not recommended.

Validating Timestamp format yyyy-MM-dd'T'HH:mm:ssZ in java?

It tried changing it to the following, but this results in a date w/o the 'Z'.

public class DateTimeXmlAdapter extends XmlAdapter<String, DateTime> {
    private static final DateTimeFormatter formatter = ISODateTimeFormat.dateHourMinuteSecond().withZoneUTC();

    @Override
    public DateTime unmarshal(String value) {
        return formatter.parseDateTime(value);
    }

    @Override
    public String marshal(DateTime value) {
        return formatter.print(value);
    }
}

Output:

2014-10-17T18:50:43

The endpoint I'm talking to says they need the 'Z' because it indicates zero offset from UTC. What's the proper way to add a 'Z' at the end when using Joda Time and JAXB?

Thanks,

Matt