Fixes an issue reported by Amy Masson where field values are not rendered for an existing member during checkout.

Fixes #14

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Changes proposed in this Pull Request:

Resolve issue where custom user fields were not rendering current values for current user on checkout page. The code supported this but the PMPro actions were never updated to pass current user context. The PR just removes that expectation and calls current user information directly.

How to test the changes in this Pull Request:

1. Have a PMPro Member pod with a group that displays fields on a checkout section + profile area and add custom fields to use.
2. Log in as a user with an existing membership (or create a new one) and set values for their custom fields.
3. For a user that has a membership level that is due for renewal go to the user's account page and click on the "Renew" link.
4. On the checkout page inspect the custom user field that is created in pods and confirm that you see the current field values rendering in the Pods fields displayed.
5. Make changes or leave the field values as-is for the Pods fields.
6. Complete the checkout process.
7. Confirm on member profile that the field values were saved as expected.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you successfully run tests with your changes locally?

Changelog entry

BUG FIX: Resolve issue where custom user fields were not rendering current values for current user on checkout page. (Props to @sc0ttkclark)