public AbstractClientOptionsBuilder addHeader(CharSequence name, Object value) {
        requireNonNull(name, "name");
        requireNonNull(value, "value");
        headers.addObject(HttpHeaderNames.of(name), value);
        return this;
    }
Adds the default HTTP header for an {@link HttpRequest} that will be sent by this {@link Client}.

<p>Note that the values of the default HTTP headers could be overridden if the same
{@link HttpHeaderNames} are defined in the {@link HttpRequest#headers()} or
{@link ClientRequestContext#additionalRequestHeaders()}.
core/src/main/java/com/linecorp/armeria/client/AbstractClientOptionsBuilder.java