Fix java 9 build

We removed a cast we needed to appease Java 9. I've recreated it
in simpler form and left a comment about why we need it.

core/src/test/java/org/elasticsearch/search/sort/AbstractSortTestCase.java

@@ -249,7 +249,9 @@ public abstract class AbstractSortTestCase<T extends SortBuilder<T>> extends EST
 
     @SuppressWarnings("unchecked")
     private T copy(T original) throws IOException {
-        return copyWriteable(original, namedWriteableRegistry,
+        /* The cast below is required to make Java 9 happy. Java 8 infers the T in copyWriterable to be the same as AbstractSortTestCase's
+         * T but Java 9 infers it to be SortBuilder. */
+        return (T) copyWriteable(original, namedWriteableRegistry,
                 namedWriteableRegistry.getReader(SortBuilder.class, original.getWriteableName()));
     }
 }