core: add a `close` to InputBufferStream

Before:
`InputBufferStream.close()` does not close their buffer so the buffer will leak.

After:
Resolves #4198.
Override the `close` for closing their buffer.

core/src/main/java/io/grpc/internal/ReadableBuffers.java

@@ -327,6 +327,11 @@ public final class ReadableBuffers {
       buffer.readBytes(dest, destOffset, length);
       return length;
     }
+
+    @Override
+    public void close() throws IOException {
+      buffer.close();
+    }
   }
 
   private ReadableBuffers() {}

core/src/test/java/io/grpc/internal/ReadableBuffersTest.java

@@ -21,6 +21,7 @@ import static org.junit.Assert.assertArrayEquals;
 import static org.junit.Assert.assertEquals;
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.never;
+import static org.mockito.Mockito.times;
 import static org.mockito.Mockito.verify;
 
 import java.io.InputStream;
@@ -119,4 +120,12 @@ public class ReadableBuffersTest {
     assertEquals(2, inputStream.read(dest, /*destOffset*/ 1, /*length*/ 2));
     assertArrayEquals(new byte[]{0x00, 'h', 'e'}, dest);
   }
+
+  @Test
+  public void bufferInputStream_close_closesBuffer() throws Exception {
+    ReadableBuffer buffer = mock(ReadableBuffer.class);
+    InputStream inputStream = ReadableBuffers.openStream(buffer, true);
+    inputStream.close();
+    verify(buffer, times(1)).close();
+  }
 }