Parse delimited from test

Hi, I need to parse multiple messages from one file. In Google protocol buffers I used [parseDelimetedFrom](https://developers.google.com/protocol-buffers/docs/reference/java/com/google/protobuf/AbstractParser#parseDelimitedFrom(java.io.InputStream)). This is my attempt to implement same method wire-way.
I tried creating WireInput from source, but it always fails on second message. I assume that recreating WireInput from the same BufferedSource somehow loses its current position. 
```
 public <M extends Message> M parseDelimitedFrom(Source source, Class<M> messageClass)
      throws IOException {
    checkNotNull(source, "source");
    checkNotNull(messageClass, "messageClass");
    BufferedSource buffer = Okio.buffer(source);
    int size = WireInput.readVarint32(buffer, 0)[0];
    if (size <= 0) {
      throw new IOException("Invalid size prefix!");
    }
    MessageAdapter<M> adapter = messageAdapter(messageClass);
    return adapter.read(WireInput.newInstance(buffer));
  }
```
Incoming source is already BufferdSource created from file, so Okio.buffer(source) won't recreate it. Maybe I'm doing something wrong?