private void copyIfNecessary() {
            if (copyOnModify) {
                copyOnModify = false;
                headers = copyHeaders();
            }
        }
Replaces all existing {@link LazyHeaderFactory LazyHeaderFactorys} for the given key
with the given {@link LazyHeaderFactory}.

<p> If the given value is {@code null}, the header at the given key will be removed. </p>
core/src/main/java/com/bumptech/glide/load/model/LazyHeaders.java