This means you'll always get the most optimal amount of allocations,
since you won't re-allocate `s`.

Let's look through a couple cases as an illustration:
  1. dt: &str   = valid date => no String allocations
  2. dt: String = valid date => no String allocations, dt moved out of
  3. dt: &str   = invalid date => 1 String allocation + the ones from
                                  sanitize_rfc822_like_date()
  4. dt: String = invalid date => 0 String allocations + the ones from
                                  sanitize_rfc822_like_date(),
                                  dt moved out of

In short, we've reduced the required amount of allocations to the
minimum

Thanks, Shepmaster
  https://chat.stackoverflow.com/transcript/62927?m=39498746#39498746