Fix string data in JST

[marocchino]

GMO API returns date value as JST string. I use 'tokyo' time zone in my rails.
But the changed time is utc So this difference causes a time lag in my app.

If incompatible with the existing operation is problem, you may want to let the user select the other in the configuration options something like this:

# config
GMO::PG.timezone = 'UTC' # or 'JST'

[marocchino]

maybe we don't need to_i here.
just return Time type will be break nothing.

The timestamp should be explicitly changed to Time in Rails, which is quite painful.

[1] pry(main)> pr = Product.first
[2] pry(main)> pr.update(updated_at: 1234567890)
   (0.2ms)  BEGIN
  Product Update (1.0ms)  UPDATE "products" SET "updated_at" = $1 WHERE "products"."id" = $2  [["updated_at", nil], ["id", 4729]]
   (0.2ms)  ROLLBACK
ActiveRecord::NotNullViolation: PG::NotNullViolation: ERROR:  null value in column "updated_at" violates not-null constraint