http_send() blocks at connect when using ngx.socket

When I test DSN with lua command line, I can successfully send message to Sentry server. But when running in open-resty, I cannot send any message to Sentry. Adding logs to raven.lua, I found the code always blocks when calling `sock:connect`:

```
ngx.log(ngx.ERR, "after tcp, before connect", self.host, self.port)
ok, err = sock:connect(self.host, self.port)
if not ok then
return nil, err
end
ngx.log(ngx.ERR,` "after connect, before send")
```

The second log never gots printed.

What could be wrong?