test(context): assert case-insensitive header names in response helpers

[yusukebe]

The author should do the following, if applicable

• Add tests
• Run tests
• bun run format:fix && bun run lint:fix to format the code
• Add TSDoc/JSDoc to document the code

[github-actions]

HTTP Performance Benchmark

Framework	Runtime	Average	Ping	Query	Body
hono (origin/main)	bun	32,929.59	45,801.38	29,419.91	23,567.47
hono (current)	bun	33,153.70	46,139.40	29,500.08	23,821.63
Change		+0.68%	+0.74%	+0.27%	+1.08%

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.08%. Comparing base (b2ae3a2) to head (85900b7).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5116   +/-   ##
=======================================
  Coverage   79.08%   79.08%           
=======================================
  Files         154      154           
  Lines       10764    10764           
  Branches     2254     2254           
=======================================
  Hits         8513     8513           
  Misses       2251     2251           

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