use 'normal closure' close code by default, fixes #127

SocketRocket uses the invalid close code `-1` as the value in the default close method. In addition to being invalid, it causes issues when communicating with some libraries. See section 11.7 WebSocket Close Code Number Registry in the [WebSocket Protocol RFC 6455](http://datatracker.ietf.org/doc/rfc6455/?include_text=1) for more detail.

```
Status Code
   The Status Code denotes a reason for a WebSocket connection
   closure as per Section 7.4 of this document.  The status code is
   an integer number between 1000 and 4999 (inclusive).
```
Can you use `SRStatusCodeNormal` for this rather than the raw value?