In #18721 we removed the discard key from the session hash used to flash
messages and that broke compatibility with Rails 4 applications because they
try to map in the discarded flash messages and it returns nil.
Fixes#24726.
Refactor FlashHash to take values for its ivars in the constructor, to pretty up FlashHash.from_session_value.
Remove stale comment on FlashHash: it is no longer Marshaled in the session so we can change its implementation.
Remove blank lines I introduced in controller/test_case.rb.
Unit tests for FlashHash#to_session_value.
Put in a compatibility layer to accept FlashHash serializations from Rails 3.0+.
Test that Rails 3.2 session flashes are correctly converted to the new format.
Remove code path for processing Rails 3.0 FlashHashes since they can no longer deserialize.
Fix session['flash'] deletion condition: it will never be empty?, it will either be nil or a hash with 'discard' and 'flashes' keys.
