java 8 compliance

Author: mredolatti

client/src/main/java/io/split/client/impressions/HttpImpressionsSender.java

@@ -69,7 +69,7 @@ public void postImpressionsBulk(List<TestImpressions> impressions) {
         try {
             HttpEntity entity = Utils.toJsonEntity(impressions);
             Map<String, List<String>> additionalHeaders = Collections.singletonMap(IMPRESSIONS_MODE_HEADER,
-                    List.of(_mode.toString()));
+                    Collections.singletonList(_mode.toString()));
             SplitHttpResponse response = _client.post(_impressionBulkTarget, entity, additionalHeaders);
 
             if (response.statusCode() < HttpStatus.SC_OK || response.statusCode() >= HttpStatus.SC_MULTIPLE_CHOICES) {

client/src/test/java/io/split/service/HttpSplitClientTest.java

@@ -45,7 +45,8 @@ public void testGetWithSpecialCharacters() throws URISyntaxException, Invocation
         RequestDecorator decorator = new RequestDecorator(null);
 
         SplitHttpClient splitHtpClient = SplitHttpClientImpl.create(httpClientMock, decorator, "qwerty", metadata());
-        Map<String, List<String>> additionalHeaders = Collections.singletonMap("AdditionalHeader", List.of("add"));
+        Map<String, List<String>> additionalHeaders = Collections.singletonMap("AdditionalHeader",
+                Collections.singletonList("add"));
 
         SplitHttpResponse splitHttpResponse = splitHtpClient.get(rootTarget,
                 new FetchOptions.Builder().cacheControlHeaders(true).build(), additionalHeaders);
@@ -119,7 +120,8 @@ public void testPost() throws URISyntaxException, IOException, IllegalAccessExce
                         KeyImpression.fromImpression(new Impression("k2", null, "t2", "on", 123L, "r1", 456L, null)),
                         KeyImpression.fromImpression(new Impression("k3", null, "t2", "on", 123L, "r1", 456L, null)))));
 
-        Map<String, List<String>> additionalHeaders = Collections.singletonMap("AdditionalHeader", List.of("add"));
+        Map<String, List<String>> additionalHeaders = Collections.singletonMap("AdditionalHeader",
+                Collections.singletonList("add"));
         SplitHttpResponse splitHttpResponse = splitHtpClient.post(rootTarget, Utils.toJsonEntity(toSend),
                 additionalHeaders);