Update jwt#shopify_user_id to return an integer

[rezaansyed]

It seems that we've introduced a slight bug for developers who are developing with the shopify_app gem.

Now that core returns the sub field of a jwt as a string, the following validation fails for any app that is developing with online access tokens using shopify_app:

shopify_app/app/controllers/shopify_app/callback_controller.rb

Line 87 in 2401342

auth_hash && jwt_shopify_domain == shop_name && jwt_shopify_user_id == associated_user_id

In this scenario, the associated_user_id parsed by omniauth remains an Integer, but the jwt_shopify_user_id parsed by our middleware is now a String. There is a type mismatch. The easiest solution for this is to cast the sub field as an integer in our middleware to ensure that everything still works with omniauth as intended (I think this is fine because it's the developer's prerogative to decide what to do with the sub field).

This issue was flagged by Sneha here: https://shopify.slack.com/archives/CG71G24BZ/p1604674034310200

Before submitting the PR, please consider if any of the following are needed:

• Update CHANGELOG.md if the changes would impact users
• Update README.md, if appropriate.
• Update any relevant pages in docs/, if necessary
• For security fixes, the Disclosure Policy must be followed.