private Map<String, List<LazyHeaderFactory>> copyHeaders() {
            Map<String, List<LazyHeaderFactory>> result =
                  new HashMap<String, List<LazyHeaderFactory>>(headers.size());
            for (Map.Entry<String, List<LazyHeaderFactory>> entry : headers.entrySet()) {
                result.put(entry.getKey(), new ArrayList<LazyHeaderFactory>(entry.getValue()));
            }
            return result;
        }
Returns a new immutable {@link LazyHeaders} object.
core/src/main/java/com/bumptech/glide/load/model/LazyHeaders.java