=htc Parse Sunday dates in Set-Cookie headers

The `Set-Cookie` header would always fail to parse when given a an expires date for a Sunday. I've updated the parsing code for both "Sun" and "Sunday", however I have only managed to test "Sun". I was never able to get an expires value for "Sunday" to parse, even though the HTTP-date parsing code seems to suggest that long day names are supported.

Cc @sirthias, @jrudolph.
No, it must be zero (see https://github.com/akka/akka/blob/release-2.3-dev/akka-http-core/src/main/scala/akka/http/util/DateTime.scala#L151).