Add adapter to support java spring session(saved session values as hash)

[wangjingling]

though pulled it, i don't make sure whether this function fit for this gem or not
pull request

sample config:
Rails.application.config.session_store :redis_session_store, { key: 'SESSION', redis: { expire_after: 30.minutes, key_prefix: 'spring:session:sessions:', hashkey_prefix: 'sessionAttr:', client: $redis, }, serializer: :json, adapter: :java_spring }