@Override
  public long writeTo(WritableByteChannel channel) throws IOException {
    long written = channel.write(buf.nioBuffer());
    buf.skipBytes((int) written);
    return written;
  }
A mock implementation for {@link Send} that returns non-null value for {@link #content()} method.
ambry-network/src/test/java/com/github/ambry/network/http2/MockSend.java