Add tip about using RDoc's --pipe command to quickly test the output of
a piece of documentation.

This can be useful for trying out combinations of markup and code, as
discussed in rails/rails@225bcad
Renamed session_serializer option to cookies_serializer
Just very so slightly better test coverage
Added HybridSerializer to upgrade existing marshal cookies (wip: need tests)
Removed an old test
Tests for the HybridSerializer
Fixed minor typo in test code
Changed the tests to ensure HybridSerializer actually migrates the cookies (currently failing)
Convert FlashHash in a Hash with indifferent access
Stringify the incoming hash in FlashHash

Stringify the incoming as well to handle incoming symbol keys from
marshalled sessions
Re-write legacy (marshal) cookies on read
Remove serializer option from session_store.rb template
Fix AppGeneratorTest: serializer option was removed from session_store
Fixed broken flash tests
Missed FlashHash#replace
Added changelog entry for Flash changes [ci skip]
Updated CHANGELOG, docs, guides and release notes.

Also added a `cookies_serializer.rb` initializer to the app template.
Migrate hash-based cookie values correctly
rm warning about variable shadowing
add a send so `apply` can be called.  Fixes #13510

THIS IS A HUGE HACK.  Thor does not allow us to define public methods
without turning them in to "thor tasks".  That means we cannot subclass
the `apply` method and make it public, so we have to make the method
private and call `send` on it.