Thank you for the fix.

I think both conditions need to be tested. Something like:

```
req.logger.debug "wwwhisper #{message}" if (req.respond_to?(:logger) && req.logger)
```

In new versions of Rack, `logger` method is present, so `req.respond_to?` returns `true`. But logging may not be enabled, in which case `req.logger` returns `nil` and `req.logger.debug` fails.  Adding the second check should fix the tests: https://travis-ci.org/wrr/rack-wwwhisper/builds/26430960 

Can you update the PR?