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.
I think a best effort should be made to keep this field final. For what I see, the initialization below relies solely on static attributes which means you may be able to initialize this in a static block. This might not be true if any of those static fields are mutable. Because of that, I'm not saying you should change this, but you should take a look at it