Improve test coverage for res.set() edge cases

[RlxChap2]

Summary

Add edge case tests for res.set() to ensure headers behave correctly with uncommon inputs.

Motivation

Some header scenarios were not explicitly tested, such as empty values, case-insensitive field names, and overwriting existing headers. These tests document and validate the current behavior.

Changes

• Test empty string header values
• Test case-insensitive header names
• Test overwriting existing header values

Testing

• All tests pass with npm test

Notes

No functional changes; tests only.

[IamLizu]

Thanks for the contribution. I am mostly 👍 on this.

One caveat: this adds an assertion that preserves current implicit Content-Type normalization behavior (text/plain -> text/plain; charset=utf-8). That is correct today, but may conflict with the direction discussed in #7145 / #7146 for 6.x.

If we want to preserve current res.set() semantics on this line, this looks good to merge. If the 6.x pass-through direction is preferred, this test should be revised accordingly.

[RlxChap2]

Thanks for the review. These tests are intended to document and validate the current res.set() behavior, including edge cases with uncommon header inputs. They do not change functionality, just ensure existing behavior is covered.