Allow MessageAggregator to disallow non-empty content

Motivation:

A user sometimes just want the aggregated message has no content at
all. (e.g. A user only wants HTTP GET requests.)

Modifications:

- Do not raise IllegalArgumentException even if a user specified
  the maxContentLength of 0

Result:

A user can disallow a message with non-empty content.

codec/src/main/java/io/netty/handler/codec/MessageAggregator.java

@@ -79,8 +79,8 @@ public abstract class MessageAggregator<I, S, C extends ByteBufHolder, O extends
     }
 
     private static void validateMaxContentLength(int maxContentLength) {
-        if (maxContentLength <= 0) {
-            throw new IllegalArgumentException("maxContentLength must be a positive integer: " + maxContentLength);
+        if (maxContentLength < 0) {
+            throw new IllegalArgumentException("maxContentLength: " + maxContentLength + " (expected: >= 0)");
         }
     }