Allow "release on close" for DataBuffer.asInputStream

This commit introduces DataBuffer.asInputStream(boolean), that can
release the underlying buffer when the stream is closed.

Furthermore, this commit adds additional javadoc.

Issue: SPR-16444

spring-core/src/test/java/org/springframework/core/io/buffer/DataBufferTests.java

@@ -182,6 +182,27 @@ public class DataBufferTests extends AbstractDataBufferAllocatingTestCase {
 		release(buffer);
 	}
 
+	@Test
+	public void inputStreamReleaseOnClose() throws IOException {
+		DataBuffer buffer = createDataBuffer(3);
+		byte[] bytes = {'a', 'b', 'c'};
+		buffer.write(bytes);
+
+		InputStream inputStream = buffer.asInputStream(true);
+
+		try {
+			byte[] result = new byte[3];
+			int len = inputStream.read(result);
+			assertEquals(3, len);
+			assertArrayEquals(bytes, result);
+		} finally {
+			inputStream.close();
+		}
+
+		// AbstractDataBufferAllocatingTestCase.LeakDetector will verify the buffer's release
+
+	}
+
 	@Test
 	public void outputStream() throws IOException {
 		DataBuffer buffer = createDataBuffer(4);