Created RestHighLevelClient for Elasticsearch won't use SSL - spring-boot

Question

I am trying to connect to an existing Elasticsearch instance. The instance is set up and working fine, which I can confirm by accessing it through the browser under https://localhost:9200. I am using Spring Data Elasticsearch version 4.1.2 and I am following the official instructions from https://docs.spring.io/spring-data/elasticsearch/docs/current/reference/html/#elasticsearch.clients.rest.
Unfortunately, when starting the application, the initial health checks of the elasticsearch instance fail, because the code is trying to use http instead of https:
o.s.w.r.f.client.ExchangeFunctions : [377e90b0] HTTP HEAD http://localhost:9200/
Therefore I added the ".usingSsl()" to the ClientConfiguration, but it does not seem to have any effect and I just don't understand why. The "funny" thing is, if I implement the reactive client as described here (https://docs.spring.io/spring-data/elasticsearch/docs/current/reference/html/#elasticsearch.clients.reactive), https suddenly works and the applications starts perfectly fine. However, I want to use the regular high level client.
Can you help me change the configuration in a way so that it finally makes use of https instead of http?
#Configuration
class ElasticsearchClientConfig: AbstractElasticsearchConfiguration() {
#Bean
override fun elasticsearchClient(): RestHighLevelClient {
val clientConfiguration = ClientConfiguration.builder()
.connectedTo("localhost:9200")
.usingSsl()
.build();
return RestClients.create(clientConfiguration).rest();
}
}