public Buffer copyTo(OutputStream out) throws IOException {
    return copyTo(out, 0, size);
  }
@Test public void copyTo() throws Exception {
    Buffer source = new Buffer();
    source.writeUtf8("party");

    Buffer target = new Buffer();
    source.copyTo(target, 1, 3);

    assertEquals("art", target.readUtf8());
    assertEquals("party", source.readUtf8());
  }