Avoid rescue nil in the default throttled response

It has a couple of cons:

1. If we slip a typo in the whole line, we won't easily catch it.  Can
   you guys spot the problem problem in the following line? Chasing such
   issues is quite tricky.

   ```ruby
   retry_after = evn['rack.attack.match_data'][:period] rescue nil
   ```

2. Throwing and catching an exception is quite slower than a new hash
   allocation, so there is a speed benefit too.

We are guaranteed from Rack that env is a `Hash`, so we can even use
`Hash#fetch`.

```ruby
retry_after = env.fetch('rack.attack.match_data', {})[:period]
```

This reads better, but always allocates the default value hash, when the
other version allocates it only when needed. If you prefer `Hash#fetch`,
I'm fine with that, as long as we avoid `rescue nil`.