AsyncMiddleManServlet.Transparent class wrongly extends ProxyServlet

`AsyncMiddleManServlet.Transparent` class should extend `AsyncMiddleManServlet`
so the consumers can override the `newServerResponseContentTransformer` &
`newServerResponseContentTransformer` to provide their own ContentTransformer.

The way it is structured right now makes it impossible to use
`AsyncMiddleManServlet.Transparent` servlet as a middle man servlet.

Signed-off-by: Assim Deodia <assim.deodia@gmail.com>

jetty-proxy/src/main/java/org/eclipse/jetty/proxy/AbstractProxyServlet.java

@@ -695,11 +695,11 @@ public abstract class AbstractProxyServlet extends HttpServlet
      */
     protected static class TransparentDelegate
     {
-        private final ProxyServlet proxyServlet;
+        private final AbstractProxyServlet proxyServlet;
         private String _proxyTo;
         private String _prefix;
 
-        protected TransparentDelegate(ProxyServlet proxyServlet)
+        protected TransparentDelegate(AbstractProxyServlet proxyServlet)
         {
             this.proxyServlet = proxyServlet;
         }

jetty-proxy/src/main/java/org/eclipse/jetty/proxy/AsyncMiddleManServlet.java

@@ -234,7 +234,7 @@ public class AsyncMiddleManServlet extends AbstractProxyServlet
      *
      * @see org.eclipse.jetty.proxy.AbstractProxyServlet.TransparentDelegate
      */
-    public static class Transparent extends ProxyServlet
+    public static class Transparent extends AsyncMiddleManServlet
     {
         private final TransparentDelegate delegate = new TransparentDelegate(this);
 

jetty-proxy/src/test/java/org/eclipse/jetty/proxy/AsyncMiddleManServletTest.java

@@ -89,6 +89,8 @@ import org.junit.Test;
 public class AsyncMiddleManServletTest
 {
     private static final Logger LOG = Log.getLogger(AsyncMiddleManServletTest.class);
+    private static final String PROXIED_HEADER = "X-Proxied";
+
     @Rule
     public final TestTracker tracker = new TestTracker();
     private HttpClient client;
@@ -1483,6 +1485,41 @@ public class AsyncMiddleManServletTest
         Assert.assertEquals(chunk1.capacity() + chunk2.capacity(), response.getContent().length);
     }
 
+    @Test
+    public void testTransparentProxyWithIdentityContentTransformer() throws Exception
+    {
+        final String target = "/test";
+        startServer(new HttpServlet()
+        {
+            @Override
+            protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException
+            {
+                if (req.getHeader("Via") != null)
+                    resp.addHeader(PROXIED_HEADER, "true");
+                resp.setStatus(target.equals(req.getRequestURI()) ? 200 : 404);
+            }
+        });
+        final String proxyTo = "http://localhost:" + serverConnector.getLocalPort();
+        AsyncMiddleManServlet proxyServlet = new AsyncMiddleManServlet.Transparent() {
+            @Override
+            protected ContentTransformer newServerResponseContentTransformer(HttpServletRequest clientRequest, HttpServletResponse proxyResponse, Response serverResponse) {
+                return ContentTransformer.IDENTITY;
+            }
+        };
+        Map<String, String> initParams = new HashMap<>();
+        initParams.put("proxyTo", proxyTo);
+        startProxy(proxyServlet, initParams);
+        startClient();
+
+        // Make the request to the proxy, it should transparently forward to the server
+        ContentResponse response = client.newRequest("localhost", proxyConnector.getLocalPort())
+                .path(target)
+                .timeout(5, TimeUnit.SECONDS)
+                .send();
+        Assert.assertEquals(200, response.getStatus());
+        Assert.assertTrue(response.getHeaders().containsKey(PROXIED_HEADER));
+    }
+
     private Path prepareTargetTestsDir() throws IOException
     {
         final Path targetTestsDir = MavenTestingUtils.getTargetTestingDir().toPath();