Check wc_checkout_params.is_checkout against string '1' instead of int 1

In checkout.js,  `wc_checkout_params.is_checkout` was being strict compared to `1`, but it is a string (`'1'`).

This was causing `'init_checkout'` not to be triggered on page load. I
noticed this when the taxes for an order weren’t being shown on the
checkout page until changing the billing address.

Introduced in b85b1ab03bd5974bf2fa7ae01580faff49b2559e

Steps to reproduce:
- Add an item to your cart
- Go to the checkout page
- The order details won't be updated until you tab through some of the billing address fields

Expected result:
- The order details should be refreshed on page load

Note: I'm not sure if I'm supposed to regenerate the `.min` file, or how to do it, so I guess I'll leave that up to you.