code: @Test public void inputStreamFromSource() throws Exception {
    OkBuffer source=new OkBuffer();
    source.writeUtf8("a");
    source.writeUtf8(repeat('b',Segment.SIZE));
    source.writeUtf8("c");
    InputStream in=new RealBufferedSource(source).inputStream();
    assertEquals(0,in.available());
    assertEquals(Segment.SIZE + 2,source.size());
    assertEquals('a',in.read());
    assertEquals(Segment.SIZE - 1,in.available());
    assertEquals(2,source.size());
    byte[] data=new byte[Segment.SIZE * 2];
    assertEquals(Segment.SIZE - 1,in.read(data,0,data.length));
    assertEquals(repeat('b',Segment.SIZE - 1),new String(data,0,Segment.SIZE - 1,UTF_8));
    assertEquals(2,source.size());
    assertEquals('b',in.read());
    assertEquals(1,in.available());
    assertEquals(0,source.size());
    assertEquals('c',in.read());
    assertEquals(0,in.available());
    assertEquals(0,source.size());
    assertEquals(-1,in.read());
    assertEquals(0,source.size());
  }
  