This commit introduces 2 new public methods in HttpHeaders in order
to leverage Java 8 ZonedDateTime in addition to the existing long
(with GMT time zone implied) variants:
- ZonedDateTime getFirstZonedDateTime(String headerName)
- void setZonedDateTime(String headerName, ZonedDateTime date)
This commit also leverages Java 8 thread-safe DateTimeFormatter for
HttpHeader implementation instead of SimpleDateFormat. As a consequence
of the usage of DateTimeFormatter.RFC_1123_DATE_TIME, HTTP date header
serialization could change slightly for single digit days from for
example "Thu, 01 Jan 1970 00:00:00 GMT" to
"Thu, 1 Jan 1970 00:00:00 GMT".
Issue: SPR-15661
5c1d8c7c59