Add ByteBuf leak detection @Rule

This commit introduces a JUnit rule that detects ByteBuf leaks in
subclasses of AbstractDataBufferAllocatingTestCase.

spring-core/src/main/java/org/springframework/core/io/buffer/NettyDataBufferFactory.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2016 the original author or authors.
+ * Copyright 2002-2017 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -49,6 +49,12 @@ public class NettyDataBufferFactory implements DataBufferFactory {
 		this.byteBufAllocator = byteBufAllocator;
 	}
 
+	/**
+	 * Return the {@code ByteBufAllocator} used by this factory.
+	 */
+	public ByteBufAllocator getByteBufAllocator() {
+		return this.byteBufAllocator;
+	}
 
 	@Override
 	public NettyDataBuffer allocateBuffer() {