Elasticsearch: keep path from base URI (#3042)

* build endpoint path with DSL
* conditional slash

elasticsearch/src/main/scala/akka/stream/alpakka/elasticsearch/impl/ElasticsearchSimpleFlowStage.scala

@@ -6,7 +6,6 @@ package akka.stream.alpakka.elasticsearch.impl
 
 import akka.annotation.InternalApi
 import akka.http.scaladsl.HttpExt
-import akka.http.scaladsl.model.Uri.Path
 import akka.http.scaladsl.model._
 import akka.http.scaladsl.unmarshalling.Unmarshal
 import akka.stream.alpakka.elasticsearch._
@@ -71,15 +70,17 @@ private[elasticsearch] final class ElasticsearchSimpleFlowStage[T, C](
     override def onPull(): Unit = tryPull()
 
     override def onPush(): Unit = {
-      val endpoint = if (settings.allowExplicitIndex) "/_bulk" else s"/${elasticsearchParams.indexName}/_bulk"
+      val endpoint =
+        if (settings.allowExplicitIndex) baseUri.path ?/ "_bulk"
+        else baseUri.path ?/ elasticsearchParams.indexName / "_bulk"
       val (messages, resultsPassthrough) = grab(in)
       inflight = true
       val json: String = restApi.toJson(messages)
 
       log.debug("Posting data to Elasticsearch: {}", json)
 
       if (json.nonEmpty) {
-        val uri = baseUri.withPath(Path(endpoint))
+        val uri = baseUri.withPath(endpoint)
         val request = HttpRequest(HttpMethods.POST)
           .withUri(uri)
           .withEntity(HttpEntity(NDJsonProtocol.`application/x-ndjson`, json))