Builder addHeader(@NonNull String name, @NonNull String value) {
            List<String> header = headers.get(name);
            if (header == null) {
                header = new ArrayList<>(1);
                headers.put(name, header);
            }
            header.add(value);
            return this;
        }
Create a new builder for this header, the new builder will do a shallow copy of this header
part.

@return A Builder to generate a header part instance.
netbare-injector/src/main/java/com/github/megatronking/netbare/http/HttpHeaderPart.java