test: assert compression with Cache-Control lacking no-transform

[renanmpimentel]

What changed

Adds a test block next to the existing Cache-Control: no-transform tests asserting that a response is still compressed when it has a Cache-Control header that does not contain no-transform (no-cache and public, max-age=60).

Why

The Cache-Control tests only cover the negative side of shouldTransform(): every case sets a value containing no-transform and asserts that the response is not compressed. Nothing checks the positive side, so a regression that disables compression for any response carrying Cache-Control goes unnoticed.

For example, either of these changes to shouldTransform() in index.js leaves the whole suite green (63 passing):

-  return !cacheControl ||
+  return !cacheControl &&
     !cacheControlNoTransformRegExp.test(cacheControl)

-  return !cacheControl ||
-    !cacheControlNoTransformRegExp.test(cacheControl)
+  return !cacheControl

That would silently stop compressing a very common class of responses, including the Server-Sent Events example in the README, which sets Cache-Control: no-cache and relies on compression + res.flush().

Verification

• npm test: 65 passing, 1 pending (was 63 passing, 1 pending)
• npm run lint: no issues

Before / after against the regression above (|| → && in shouldTransform):

Suite	Correct code	With regression
Before this PR	63 passing	63 passing (regression undetected)
After this PR	65 passing	2 failing: expected "Content-Encoding" header field

The second variant (return !cacheControl) gives the same result.

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I found this while experimenting with Supertest (unrelated to the supertest HTTP library), a tool for evaluating test effectiveness using mutation testing and test-harness mutilation.