revert serializer changes

@dennisreimann sorry for causing such a mess, should have tested that properly earlier ...
Turns out our non-json detector had a bug and did not catch the nested current/services in the serialized hash, fixing this would mean iterating over current/services and serializing+deserializing every element, which is just a mess (tried it, makes the code grow about 15 loc + ugly)

also FYI I realized that json session is not going to happen since rails flash uses an array of symbols internally which cannot be jsonified, so I'm killing that project :)