Fix buffer leak in HttpObjectEncoder when message has no content (#12828

)

Motivation:

When the _HttpObjectEncoder.encodeAndClose_ method encodes a response which content is _always empty_ (as described in [_HttpObjectEncoder. isContentAlwaysEmpty](https://github.com/netty/netty/blob/main/codec-http/src/main/java/io/netty5/handler/codec/http/HttpObjectEncoder.java#L240) ), then the empty last http content or the empty FullHttpResponse is not closed.

An http response content is always empty for example when status code is INFORMATIONAL, is a 204, 205, 304 ... [See for example here](https://github.com/netty/netty/blob/main/codec-http/src/main/java/io/netty5/handler/codec/http/HttpResponseEncoder.java#L67)

Example scenario: 

1) a _DefaultHttpResponse_ is first encoded with status 204, in this case, the _HttpObjectEncoder.encodeAndClose_ method will initialize its state to **ST_CONTENT_ALWAYS_EMPTY**, [see here](https://github.com/netty/netty/blob/main/codec-http/src/main/java/io/netty5/handler/codec/http/HttpObjectEncoder.java#L96).

2) next, an _EmptyLastHttpContent_ is encoded. So, the _HttpObjectEncoder.encodeAndClose_ state being in _ST_CONTENT_ALWAYS_EMPTY_, then the encodeAndClose method switch statement will proceed [here](https://github.com/netty/netty/blob/main/codec-http/src/main/java/io/netty5/handler/codec/http/HttpObjectEncoder.java#L149) directly, which is not closing the _EmptyLastHttpContent_

Modification:

Move the code which was disposing the message [from here](https://github.com/netty/netty/blob/main/codec-http/src/main/java/io/netty5/handler/codec/http/HttpObjectEncoder.java#L144) to the case on ST_CONTENT_ALWAYS_EMPTY [here](https://github.com/netty/netty/blob/main/codec-http/src/main/java/io/netty5/handler/codec/http/HttpObjectEncoder.java#L150).
Also added some test cases in _HttpResponseEncoderTest_.

Result:

Disposing the message to the case on _ST_CONTENT_ALWAYS_EMPTY_ will continue to ensure that the message is closed when the state is _ST_CONTENT_NON_CHUNK_ and when content length is zero (because the case fall-through, [see here](https://github.com/netty/netty/blob/main/codec-http/src/main/java/io/netty5/handler/codec/http/HttpObjectEncoder.java#L145)), and also ensures that any HttpContent is closed when state is _ST_CONTENT_ALWAYS_EMPTY_.

codec-http/src/main/java/io/netty5/handler/codec/http/HttpObjectEncoder.java

@@ -141,13 +141,13 @@ protected void encodeAndClose(ChannelHandlerContext ctx, Object msg, List<Object
 
                         break;
                     } else {
-                        Resource.dispose(msg);
                         // do not break, let's fall-through
                     }
 
                     // fall-through!
                 case ST_CONTENT_ALWAYS_EMPTY:
 
+                    Resource.dispose(msg);
                     if (buf != null) {
                         // We allocated a buffer so add it now.
                         out.add(buf);

codec-http/src/test/java/io/netty5/handler/codec/http/HttpResponseEncoderTest.java

@@ -396,4 +396,32 @@ private static void testStatusResetContentTransferContentLength0(CharSequence he
         assertEquals(responseText, written.toString());
         assertFalse(channel.finish());
     }
+
+    @Test
+    void testResponseNoContentWithEmptyLastContentClosed() {
+        testResponseNoContentWithLastContentClosed(new EmptyLastHttpContent(preferredAllocator()));
+    }
+
+    @Test
+    void testResponseNoContentWithDefaultLastHttpContentClosed() {
+        testResponseNoContentWithLastContentClosed(new DefaultLastHttpContent(preferredAllocator().allocate(0)));
+    }
+
+    private void testResponseNoContentWithLastContentClosed(LastHttpContent<?> lastHttpContent) {
+        EmbeddedChannel channel = new EmbeddedChannel(new HttpResponseEncoder());
+        HttpResponse response = new DefaultHttpResponse(HttpVersion.HTTP_1_1, HttpResponseStatus.NO_CONTENT);
+        assertTrue(channel.writeOutbound(response, lastHttpContent));
+        assertTrue(channel.finishAndReleaseAll());
+        assertFalse(lastHttpContent.isAccessible());
+    }
+
+    @Test
+    void testFullResponseNoContentClosed() {
+        EmbeddedChannel channel = new EmbeddedChannel(new HttpResponseEncoder());
+        FullHttpResponse response = new DefaultFullHttpResponse(HttpVersion.HTTP_1_1, HttpResponseStatus.NO_CONTENT,
+                preferredAllocator().allocate(0));
+        assertTrue(channel.writeOutbound(response));
+        assertTrue(channel.finishAndReleaseAll());
+        assertFalse(response.isAccessible());
+    }
 }