Support content type application/x-ndjson in DeprecationRestHandler

org.elasticsearch.rest.RestController#hasContentType checks to see if the
RestHandler supports the `application/x-ndjson` Content-Type. DeprecationRestHandler
is a wrapper around the real RestHandler, and prior to this change
would always return `false` due to the interface's default supportsContentStream().
This prevents API's that use multi-line JSON from properly being deprecated
resulting in an HTTP 406 error.

This change ensures that the DeprecationRestHandler honors the
supportsContentStream() of the wrapped RestHandler.

Part of elastic#35958

server/src/main/java/org/elasticsearch/rest/DeprecationRestHandler.java

@@ -62,6 +62,11 @@ public void handleRequest(RestRequest request, RestChannel channel, NodeClient c
         handler.handleRequest(request, channel, client);
     }
 
+    @Override
+    public boolean supportsContentStream() {
+        return handler.supportsContentStream();
+    }
+
     /**
      * This does a very basic pass at validating that a header's value contains only expected characters according to RFC-5987, and those
      * that it references.

server/src/test/java/org/elasticsearch/rest/DeprecationRestHandlerTests.java

@@ -28,6 +28,7 @@
 
 import static org.mockito.Mockito.inOrder;
 import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
 
 /**
  * Tests {@link DeprecationRestHandler}.
@@ -114,6 +115,13 @@ public void testInvalidHeaderValueEmpty() {
         expectThrows(IllegalArgumentException.class, () -> DeprecationRestHandler.requireValidHeader(blank));
     }
 
+    public void testSupportsContentStream() {
+        DeprecationRestHandler deprecationRestHandler = new DeprecationRestHandler(handler, deprecationMessage, deprecationLogger);
+        when(handler.supportsContentStream()).thenReturn(true).thenReturn(false);
+        assertTrue(deprecationRestHandler.supportsContentStream());
+        assertFalse(deprecationRestHandler.supportsContentStream());
+    }
+
     /**
      * {@code ASCIIHeaderGenerator} only uses characters expected to be valid in headers (simplified US-ASCII).
      */