V5 fix cors headers reflection

[kwhitley]

Description

FIXES:

• Corrects the included CORS headers, and their distribution between preflight and final responses
• Reflects requested-headers as allowed-headers by default, unless specified (via options)

Type of Change (select one and follow subtasks)

• Documentation (README.md)
• Maintenance or repo-level work (e.g. linting, build, tests, refactoring, etc.)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
  • I have included test coverage
• Breaking change (fix or feature that would cause existing functionality/userland code to not work as expected)
  • Explain why a breaking change is necessary:
• This change requires (or is) a documentation update
  • I have added necessary local documentation (if appropriate)
  • I have added necessary itty.dev documentation (if appropriate)