At present, MockCookie doesn't preserve expires attribute. This has a consequence that cookie value set using MockHttpServletResponse#addHeader containing expires attribute will not match the cookie value obtained from MockHttpServletResponse#getHeader since the expires will get calculated based on current time.

This is very simple to demonstrate using a test that's added to MockHttpServletResponseTests as a part of this PR:

@Test
void addCookieHeaderWithExpires() {
    String cookieValue = "SESSION=123; Path=/; Max-Age=100; Expires=Tue, 8 Oct 2019 19:50:00 GMT; Secure; " +
            "HttpOnly; SameSite=Lax";
    response.addHeader(HttpHeaders.SET_COOKIE, cookieValue);
    assertThat(response.getHeader(HttpHeaders.SET_COOKIE)).isEqualTo(cookieValue);
}

This PR enhances MockCookie to preserve expires attribute.