Change AggregatedFullHttpMessage to contain a content ByteBuf

Motivation:

Other implementations of FullHttpMessage allow .toString to be called after the Message has been released
This brings AggregatedFullHttpMessage into line with those impls.

Modifications:

- Changed AggregatedFullHttpMessage to no longer be a sub-class of DefaultByteBufHolder
- Changes AggregatedFullHttpMessage to implement ByteBufHolder
- Hold the content buffer internally to AggregatedFullHttpMessage
- Implement the required content() and release() methods that were missing
- Do not check refcnt when accessing content() (similar to DefaultFullHttpMessage)

Result:

A released AggregatedFullHttpMessage can have .toString called without throwing an exception

codec-http/src/main/java/io/netty/handler/codec/http/HttpObjectAggregator.java

@@ -16,6 +16,7 @@
 package io.netty.handler.codec.http;
 
 import io.netty.buffer.ByteBuf;
+import io.netty.buffer.ByteBufHolder;
 import io.netty.buffer.CompositeByteBuf;
 import io.netty.buffer.DefaultByteBufHolder;
 import io.netty.buffer.Unpooled;
@@ -277,13 +278,14 @@ public class HttpObjectAggregator extends MessageToMessageDecoder<HttpObject> {
         return fullMsg;
     }
 
-    private abstract static class AggregatedFullHttpMessage extends DefaultByteBufHolder implements FullHttpMessage {
+    private abstract static class AggregatedFullHttpMessage implements ByteBufHolder, FullHttpMessage {
         protected final HttpMessage message;
+        private final ByteBuf content;
         private HttpHeaders trailingHeaders;
 
         AggregatedFullHttpMessage(HttpMessage message, ByteBuf content, HttpHeaders trailingHeaders) {
-            super(content);
             this.message = message;
+            this.content = content;
             this.trailingHeaders = trailingHeaders;
         }
 
@@ -328,17 +330,37 @@ public class HttpObjectAggregator extends MessageToMessageDecoder<HttpObject> {
         }
 
         @Override
-        public FullHttpMessage retain(int increment) {
-            super.retain(increment);
-            return this;
+        public ByteBuf content() {
+            return content;
+        }
+
+        @Override
+        public int refCnt() {
+            return content.refCnt();
         }
 
         @Override
         public FullHttpMessage retain() {
-            super.retain();
+            content.retain();
             return this;
         }
 
+        @Override
+        public FullHttpMessage retain(int increment) {
+            content.retain(increment);
+            return this;
+        }
+
+        @Override
+        public boolean release() {
+            return content.release();
+        }
+
+        @Override
+        public boolean release(int decrement) {
+            return content.release(decrement);
+        }
+
         @Override
         public abstract FullHttpMessage copy();