Check wc_checkout_params.is_checkout against string '1' instead of int 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
Merge pull request #5236 from JDGrimes/master

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