Fixing typo in guide, updating ActionMailer description and making the contributors list alphabetical

Rails on Rack Rails guide: indicate that Metal pieces now require an "X-Cascade" header with a value of "pass" to continue the Metal chain execution instead of a HTTP 404 response. Also removed reference to old code.

Rails on Rack Rails guide: indicate that Metal pieces now require an "X-Cascade" header with a value of "pass" to continue the Metal chain execution instead of a HTTP 404 response. Also removed reference to old code.

They can be bundled. Just the git repos can't. Use rails/rack-mount for that.

This reverts commit ef6cdc97.

Fixing up some errors and changing the observer pattern to a much more simple and direct controller pattern

Revert "I18n guide: change ActionController by Action Controller in some places and fix code example in 2.4 section"

This reverts commit 002da9b6.

I18n guide: change ActionController by Action Controller in some places and fix code example in 2.4 section