Prior to this change, calling the `setDateHeader` method on a
MockHttpServletResponse instance (internal implementation for testing
the spring-web module) would just store the given long value in a Map,
not writing it as a formatted date String.
This can be problematic when testing features related to date headers
such as "Expires", "If-Modified-Since", "Last-Modified", etc.
This commit formats long dates into date Strings using the date format
recommended by the RFC and the GMT time zone.
088a50c1fb