When creating a new Headers object using an existing Headers object
the existing Headers map is copied by reference. Therefore adding a
new Header value to the new Headers object also added this value to
the existing Headers object which is not in accordance with the
spec.
This commit alters the constructor to create a deep copy of existing
Headers maps and therefore unlink existing Headers from new Headers.
Closes#6845
BREAKING CHANGE:
any code which relies on the fact that a newly
created Headers object is referencing an existing Headers map is
now broken, but that should normally not be the case since this
behavior is not documented and not in accordance with the spec.
