Revert "Returns after encoding each message not do check following instance types"

Motivation:
This reverts commit 3405aee2ab. This commit introduces a bug and the encoder no longer encodes FullMemcacheMessage objects correctly.

Modifications:
- Revert commit

Result:
Fixes https://github.com/netty/netty/issues/5197

codec-memcache/src/main/java/io/netty/handler/codec/memcache/AbstractMemcacheObjectEncoder.java

@@ -45,7 +45,6 @@ public abstract class AbstractMemcacheObjectEncoder<M extends MemcacheMessage> e
             @SuppressWarnings({ "unchecked", "CastConflictsWithInstanceof" })
             final M m = (M) msg;
             out.add(encodeMessage(ctx, m));
-            return;
         }
 
         if (msg instanceof MemcacheContent || msg instanceof ByteBuf || msg instanceof FileRegion) {