WebClient throws exception if Jetty reactive client is not on classpath

Using WebClient with jetty client connector throws

```
java.lang.NoClassDefFoundError: org/eclipse/jetty/reactive/client/ReactiveRequest
```

when `org.eclipse.jetty.http2:jetty-http2-client` is on classpath and `org.eclipse.jetty:jetty-reactive-httpclient` is not on classpath.

