test: add unit tests for res.contentType alias and parameters in res.type

[thePranav-kpk]

Description

This PR adds explicit unit test coverage for:

1. The res.contentType(str) alias. In lib/response.js, res.contentType is defined as an alias for res.type(str), but it had no unit tests in test/res.type.js to ensure it works correctly and behaves identically.
2. Preserving media type parameters (like charsets or custom parameters) when passing type/subtype strings directly to res.type(str) (e.g. res.type('text/html; charset=utf-8')).

Changes

• test/res.type.js:
  • Added test case for parameter preservation when calling res.type().
  • Added a .contentType(str) test block to verify that the alias behaves identically to res.type() for file lookups, literal types, and literal types with parameters.

Checklist

• All tests pass locally (npx mocha test/res.type.js passed)
• Code conforms to ESLint formatting standards (npm run lint passed)