THis works:

package org.openqa.selenium.htmlunit;

This fixes the issue - but I would perfer it to come back into HtmlUnitDriver

public class ExtendedHtmlUnitDriver extends HtmlUnitDriver
{
    public ExtendedHtmlUnitDriver(boolean enableJavascript)
    {
        super(enableJavascript);
    }

    public void setHeader(String name, String value)
    {
        this.getWebClient().addRequestHeader(name, value);
    }
}


Usage:
        ExtendedHtmlUnitDriver driver = new ExtendedHtmlUnitDriver(true);
        driver.setHeader("cac", "2200009990");