Afform - Fix picking default value for date field

[colemanw]

Overview

Fixes dev/core#3846

Before

Unable to set default value for date field

After

Fixed

[civibot]

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[jackgleeson]

• General standards
  • Explain (r-explain)
    • PASS : The goal/problem/solution have been adequately explained in the PR.
  • User impact (r-user)
    • PASS: The change would be intuitive or unnoticeable for a majority of users who work with this feature.
  • Documentation (r-doc)
    • PASS: The changes do not require documentation.
  • Run it (r-run)
    • PASS: Pulled down fix, created default form with default Birthdate field and saved successfully. I then confirmed the default date was displayed when browsing the test form. All good!
• Developer standards
  • Technical impact (r-tech)
    • PASS: The change preserves compatibility with existing callers/code/downstream.
  • Code quality (r-code)
    • PASS: The functionality, purpose, and style of the code seems clear+sensible.
  • Maintainability (r-maint)
    • PASS: The change does not sufficiently improve test coverage, but special circumstances make it important to accept the change anyway.
  • Test results (r-test)
    • PASS: The test results are all-clear.

[kurund]

thanks @jackgleeson