@Override
    public long writeTo(WritableByteChannel channel) throws IOException {
      long written = channel.write(buf.nioBuffer());
      buf.skipBytes((int) written);
      return written;
    }
A mock {@link Send} implementation that returns non-null value for {@link #content()} method.
ambry-protocol/src/test/java/com/github/ambry/protocol/RequestResponseTest.java