Prefer `array?` rather than `array`

Happy New Year! :tada: :sheep:

`array` can only be called to check to dump the `:array` option in the AR internal, therefore it is not necessary to determine until it is called. Do not remove `[]` from `sql_type` for the `array?` method. Lookup the `cast_type` from `sql_type` required `[]` also. It was not necessary to remove `[]`.
This lines has been introduced from https://github.com/rails/rails/pull/7547:

https://github.com/rails/rails/pull/7547/files#diff-cc31fc1dd1e78db64638200f710b2f59R51

However, I think that there was no reason to remove `[]`.