Added `502 Bad Gateway` to the STATUSES hash.

`502 Bad Gateway` is used in the case of upstream failure when uploading release assets.

The response was previously [documented](fcaed8d4), however the STATUSES array wasn't updated to support this response.

Before:

![screen shot 2013-10-21 at 14 04 08](https://f.cloud.github.com/assets/65057/1372022/ef94385e-3a48-11e3-8c39-201ace4767b4.png)

After:

![screen shot 2013-10-21 at 14 05 17](https://f.cloud.github.com/assets/65057/1372028/133f69d6-3a49-11e3-8c01-f7010be00651.png)

@technoweenie