@Override
    public AbstractHttpMessageBuilder header(CharSequence name, Object value) {
        headersBuilder().addObject(requireNonNull(name, "name"),
                                   requireNonNull(value, "value"));
        return this;
    }
Returns the {@link Publisher} that was set by {@link #content(MediaType, Publisher)}.
core/src/main/java/com/linecorp/armeria/common/AbstractHttpMessageBuilder.java