fix: check for default headers before appending to cors header failures

[saska]

Description

• Some clients sometimes apparently (AFAIK needlessly) check OPTIONS even if they only have CORS-safelisted headers to ask about; I ran into this with Dio. I see no reason to allow I guess?

Closes

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.35%. Comparing base (6768845) to head (7861c3a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4015   +/-   ##
=======================================
  Coverage   98.35%   98.35%           
=======================================
  Files         347      347           
  Lines       15783    15783           
  Branches     1745     1745           
=======================================
  Hits        15523    15523           
  Misses        124      124           
  Partials      136      136           

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[saska]

i.e. dio sent this OPTIONS

[github-actions]

Documentation preview will be available shortly at https://litestar-org.github.io/litestar-docs-preview/4015