issue-489: Update SearchScroll for large scroll_ids

Refs Graylog2/graylog2-server#4190

(cherry picked from commit 2a34a76)

jest-common/src/main/java/io/searchbox/core/SearchScroll.java

@@ -14,12 +14,14 @@ public class SearchScroll extends GenericResultAbstractAction {
     static final int MAX_SCROLL_ID_LENGTH = 1900;
     private final String restMethodName;
 
+
     protected SearchScroll(Builder builder) {
         super(builder);
 
         if(builder.getScrollId().length() > MAX_SCROLL_ID_LENGTH) {
             this.restMethodName = "POST";
-            this.payload = builder.getScrollId();
+            // represent scroll_id in json for request body
+            this.payload = "{\"scroll_id\":\"" + builder.getScrollId() + "\"}";
         } else {
             this.restMethodName = "GET";
         }

jest-common/src/test/java/io/searchbox/core/SearchScrollTest.java

@@ -6,6 +6,7 @@
 import org.junit.Test;
 
 import java.io.IOException;
+import java.util.Collections;
 
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertFalse;
@@ -31,11 +32,13 @@ public void methodIsGetIfScrollIdIsShort() throws IOException {
     public void methodIsPostIfScrollIdIsLong() throws IOException {
         String scrollId = Strings.padStart("scrollId", 2000, 'x');
 
+        final String expectedResults = "{\"scroll_id\":\""+scrollId+"\"}";
+
         Action searchScroll = new SearchScroll.Builder(scrollId, "1m").build();
         String uri = searchScroll.getURI();
 
         assertEquals("POST", searchScroll.getRestMethodName());
-        assertEquals(scrollId, searchScroll.getData(new ObjectMapper()));
+        assertEquals(expectedResults, searchScroll.getData(new ObjectMapper()));
         assertTrue(uri.length() < 2000);
         assertFalse(uri.contains(scrollId));
     }