Prior to this commit, the `HttpHeaders.writeableHttpHeaders` would only
consider headers read-only instances that were wrapped once by
`HttpHeaders.readOnlyHttpHeaders`. This does not work when other
`HttpHeaders` wrappers are involved in the chain.
This commit ensures that `writeableHttpHeaders` unwraps all headers
instances down to the actual multivalue map and create a new headers
instance out of it.
Fixes gh-33789
a06bbccf9e