fix(res): remove magic from res.set() Content-Type

[vaibhavmashal]

Problem

Currently, res.set() contains unexpected magic when setting the Content-Type header. If a given content-type string does not contain a / (e.g., shorthand like 'json' or invalid input), res.set() attempts a MIME type lookup via mime.contentType(). This mutates user input unexpectedly and causes a known bug (Issue #7034), where unrecognized types return false from mime.contentType() and the header gets coercively set to the literal string "false".

Solution

This PR removes the MIME type lookup logic from res.set() completely, keeping res.set() strictly for assigning header values as provided, which resolves unexpected mutations. The intended way to set a content type with MIME type expansion remains res.type() (which already exists for this exact purpose).

By migrating internal dependencies (res.json and res.jsonp) to use res.type() instead of res.set(), we maintain backward compatibility for built-in response types (such as automatically appending charset=utf-8 to JSON responses) while fulfilling the goal of making res.set() predictable.

Changes Made

• lib/response.js:
  • res.set(): Removed the mime.contentType() assignment block and the associated obsolete JSDoc comments. Kept the validation that prevents Content-Type from being set to an Array.
  • res.json(): Refactored this.set('Content-Type', ...) to this.type('json') to preserve charset=utf-8 expansion.
  • res.jsonp(): Refactored this.set('Content-Type', ...) to this.type('json') and this.type('text/javascript') to maintain expansion behavior for JSON/JSONP responses.
• test/res.send.js:
  • Updated expectations in tests where res.set('Content-Type', 'text/plain') was used, expecting exactly 'text/plain' rather than 'text/plain; charset=utf-8'.

Testing

• To reproduce the original bug:
  1. Call res.set('Content-Type', 'some-custom-type');
  2. Observe that the header sent in the response is erroneously set to Content-Type: false.
• To verify the fix:
  1. Run the same test above, and observe the output correctly maintains Content-Type: some-custom-type.
  2. Verify that res.type('json') and res.json({}) still properly resolve to application/json; charset=utf-8.
  3. Ensure all tests in the Express suite pass via npm test (all 1260 tests passed locally).

Fixes #7145
Fixes #7034

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