Check fails on sites that return HTTP 5xx codes

I tried testing a proxy server that terminates TLS/SSL but with all the backend HTTP servers down. With a regular user-agent when connecting with HTTPS you see an HTTP 502/gateway error page. When trying to test the proxy with the heartbleed checker, it handshakes TLS correctly with the proxy but returns this warning:

```
Uh-oh, something went wrong: dial tcp x.x.x.x:443: i/o timeout
```