public Builder addHeader(String key, String value) {
            return addHeader(key, new StringHeaderFactory(value));
        }
Adds a value for the given header and returns this builder.

<p> Use {@link #addHeader(String, LazyHeaderFactory)} if obtaining the value requires I/O
(ie an oauth token). </p>

@see #addHeader(String, LazyHeaderFactory)
core/src/main/java/com/bumptech/glide/load/model/LazyHeaders.java