@Nullable public String getFirst(HeaderName headerName) {
        String normalName = Objects.requireNonNull(headerName, "headerName").getNormalised();
        return getFirstNormal(normalName);
    }
Get the first value found for this key even if there are multiple. If none, then
return {@code null}.
zuul-core/src/main/java/com/netflix/zuul/message/Headers.java