Prevent to break cookie domain

CookieJar breaks the domain of a cookie when the cookie is added.
It inserts `.` at the beginning of it by calling `domain().prepend('.')` in `CookieJar::contains()`.

`CookieJar::contains()` shouldn't break any object states, so this pull-request fixes this problem by using `'.' + domain()` instead of `QString::prepend()`.