Add serializer option to cookie store and use Rails 4 Hash flash

Backport for Rails 4 flash hash based on https://github.com/envato/rails_4_session_flash_backport
Update tests to load flash from session value
indifferent access to flash hash
indifferent delete
support indifferent access for hashes stored within FlashHash
Revert nested hash indifference, swap delete order

Upstream doesnt support nested hashes having indifferent access, we
should stay consistent. Swap order for returned value in session hash.
Merge branch '2-3-github' into json-sessions
bump rails version
Merge pull request #64 from github/json-sessions

Support custom serialization for Session::CookieStore