Hstore and JSON fields are not supported with store

The use of `store` on Hstore fields (for instance) is not needed
because serialization aimed to interact with Ruby objects and not
with provided field types
exists? is deprecated in ruby trunk [ci skip]

see here 4d4ff531b8807ee88a3fc46875c7e76f613956fb
Fixed code syntax for config.ru [ci skip]
refine usage docs of `store` in combination with `hstore` and `json`.

refs #11561 #12700
Update Rails initialization guide to explain how bin/rails is executed