This change avoids the trap of creating a copy of `HttpHeaders` using a
case-sensitive `MultiValueMap` by mistake. Since mutability is always
desirable, we make a mutable copy by using `addAll` on an empty
`HttpHeaders`.
We can't simply rely on HttpHeaders' map-based constructor to detect
read-only header in this particular case, because the container's
original headers representation might in some cases be read-only.
Closes gh-33666
ef77b4064f