public void setHeaders(CaseInsensitiveStringMultiMap mm) {
        this.mm = mm;
    }
Other code will create loggers before our test runs, and we don't want to break anything,
so we'll provide an empty map here. The test itself can override the map with
{@link #setHeaders(CaseInsensitiveStringMultiMap)} to allow REST tests to reflect the results of
logging as headers (and hence make testable assertions).
gateway/test/src/test/java/io/apiman/gateway/test/logging/TestLogger.java