As defined in RFC2616 - section-13.5.1, shared caches MUST
store end-to-end headers from backend response and MUST be
transmitted in any response formed from a cache entry.

This commit ensures a stronger consistency between responses
served from the handlers & from the cache middleware.

Set flag to prevent e2e headers caching to
be the default behavior of the cache middleware.
This would otherwise change quite a lot the
experience for cache middleware current users.

E2E is an acronym commonly associated with test.
While in the present case it refers to end-to-end
HTTP headers (by opposition to hop-by-hop), this
still remains confusing. This commits renames it
to a more generic name.

This will prevent an extra memory allocation for users
not interested in this feature.

…rs is disabled.

- use set instead of add for the headers
- copy value from the headers -> prevent problems with mutable values