DroppingBuffer is broken

DroppingBuffer seems to inherit the prototype from itself, which breaks its functionality (e.g. Channel.output.put produces an error). I have fixed it as follows:

buffer.js : line 64:

```
DroppingBuffer.prototype = Object.create(DroppingBuffer.prototype)
```

should be

```
DroppingBuffer.prototype = Object.create(UnblockingBuffer.prototype)
```

I wasn't sure if this should be a pull request or not, so I decided to report it instead.