Fix MockResponse.getheaders not returning header values

[bysiber]

MockResponse.getheaders() calls self._headers.getheaders(name) but doesn't return the result. The method always returns None regardless of what headers are present.

This method is part of the interface expected by http.cookiejar.CookieJar for extracting cookies from responses. While the current cookie extraction path goes through MockResponse.info() → HTTPMessage, getheaders() is part of the public contract of MockResponse and any code relying on it would silently get None instead of the actual header values.

The fix adds the missing return statement.