jadler-core/src/main/java/net/jadler/matchers/HeaderRequestMatcher.java
@Override
    protected List<String> retrieveValue(final Request req) {
        return req.getHeaders().getValues(this.headerName);
    }
@Override
    protected List<String> retrieveValue(final Request req) {
        return req.getHeaders().getValues(this.headerName);
    }
Retrieves a header (defined in {@link #HeaderRequestMatcher(org.hamcrest.Matcher, java.lang.String)})
of the given request.
@param req request to retrieve the header from
@return the request header as a list of values or {@code null} if there is no such a header in the request