Use element operator instead of contains for jsonb querying

[shioyama]

Currently, queries on jsonb columns in the Jsonb and Container backends are performed using the "contains" operator (@>). This works fine for string/integer/boolean values, but for hash valued-translations it returns a match even if the hash contains other keys/values (i.e. it does not perform an exact match).

Instead of using "contains" (@>), we can also use the normal element operator (->), so for a query Post.i18n.where(title: { "a" => "b" }), instead of this:

SELECT "posts".* FROM "posts" WHERE ("posts"."title" @> '{"en":{"a":"b"}}')

we instead get this:

SELECT "posts".* FROM "posts" WHERE (("posts"."title" -> 'en') = '{"a":"b"}')

The first SQL will match a post with a title value '{"a":"b","c":"d"}', but the second will not.

Although this is an edge case, I think in general it's also better not to use the jsonb-specific contains operator if it's not really necessary anyway.