MULE-7774: Changing date header format for HTTP response builder and tim...

...e zone for GMT. Adding system property to maintain previous versions time zone, including general class to track such properties in the future.
Better than this, create an instance of the BackwardsCompatibilityChecker as a constant.
Add some methods to activate or deactivate programatically the property to use in tests, so that we don't have to use the system property string anywhere.