…on response middleware
Fixes, for instance, receiving responses to HEAD in Rails.
Because of the missing `Rack::Cache::Storage.instance.clear` line in test setup, some operations in the test were actually silently failing due to uninitialized cache dirs. This ensures future exceptions in Rack::Cache don't go unnoticed. Closes #28
In tests, avoid using `send` to work with dynamic HTTP methods. Instead use `new_stub` and `run_request` methods that accept HTTP method as argument.