v0.18.5 : x-www-form-urlencoded bug fix

This releases fixes a bug related to default values in the x-www-form-urlencoded encoding.

See #1323

What's Changed

• Updated headers for 2024 by @Baccata in #1337
• Bump actions/download-artifact from 3 to 4 by @dependabot in #1336
• Use field.getUnlessDefault instead of field.get in UrlFormDataEncoder by @bpholt in #1323

Full Changelog: v0.18.4...v0.18.5