public Request addHeader(final Header header) {
        this.request.addHeader(header);
        return this;
    }
Execute the request using an HTTP/2 capable engine. The exact protocol version
will still have to be negotiated by individual connections.
<p>
This feature is considered experimental and may be discontinued in the future.

@since 5.5
httpclient5-fluent/src/main/java/org/apache/hc/client5/http/fluent/Request.java