[Backport 3.2] Parsing of expires headers fails if expires is set directly on backing HttpServletResponse

In HSTTWO-3554 I made a fix to correctly parse the expires header because it was assumed to always be in RFC1123 date format. However this is only the case when the setDateHeader is invoked on a HstResponse. If setDateHeader is invoked directly on the backing HttpServletResponse, you get the long (ms since epoch) back (the container will later translate it to RFC1123)