@Override
    public Collection<List<String>> values() {
        return content.values();
    }
Convenience method for checking whether a named header field is <em>true</em>. To satisfy this, the
header field needs to have at least 1 entry, and Boolean.valueOf() of all its values must parse as
<em>true</em>.

@param key the key whose values to parse as a boolean.
@return the boolean value of the named header.
jdisc_core/src/main/java/com/yahoo/jdisc/HeaderFields.java