test: add coverage for req.is() with array argument

[roshnithakor07]

Closes #7348

Adds test coverage for req.is() when called with an array
argument directly (e.g. req.is(['html', 'json'])), which was
previously untested and showed up as an uncovered branch on
lib/request.js line 273.

Before:

• request.js branch coverage: 98.07%, line 273 uncovered

After:

• request.js: 100% across all metrics

[Ziiyodullayevv]

Good test coverage addition. Testing req.is() with array arguments ensures the method works correctly for content-type matching with multiple types.