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.
This sentence is not clear. I would rephrase as "Http Date headers will be on timezone {0}  since system property {1} specifies it should match the timezone from the running Mule Server. This property might be removed in the future, blah blabh blah"