Allow users to disable the secure cookie warning for custom coders

[dmcinnes]

Hello,

Since we've recently updated Rack we've been getting the "No secret option provided to Rack::Session::Cookie" warning in our logs. However we are providing a custom coder to Rack::Session::Cookie that handles all of the encryption:
https://github.com/heroku/identity/blob/master/lib/identity/main.rb#L12-L19

This pull request adds an option to Rack::Session::Cookie to disable this warning, but only if a coder is supplied in the cases were the user wants to manage the cookie encryption themselves:

use Rack::Session::Cookie,
  coder: MyFancyEncryptionCoder.new,
  let_coder_handle_secure_encoding: true

Does this approach make sense? If not I'm happy to hear alternatives.

Thanks!

[brandur]

Nice! Thanks @tenderlove!

[dmcinnes]

Thanks!