Edit rescue procedure in the HTTP connection part

I am using out_http plugin in our service. it has been running in so many servers.
but, sometimes, Fluentd has been broken down(looks like freezing td-agent) because it occurred http connection error.
so, I researched this trouble, and I found it. and I edited rescue procedure in the HTTP connection part in your source.

could you please confirm my pull request?

thanks.


I wonder if just catching all kinds of exceptions will be fine here. i.e.

```
begin
  # do request
rescue # catch all exceptions
  $log.warn ...
else
  unless
    ...
  end
end
```

Will this work for you?