function onReadable() {
		var chunk;
		while ( true ) {
			chunk = stream.read();
			if ( chunk === null ) {
				break;
			}
			if ( typeof chunk === 'string' ) {
				chunk = string2buffer( chunk );
			}
			data.push( chunk );
			len += chunk.length;
		}
	}
Callback invoked once the stream has data to consume.

@private