The `HttpHeaders#headerSet` method is intended as a drop-in replacement
for `entrySet` that guarantees a single casing for all header names
reported during the iteration, as the cost of some overhead but with
support for iterator removal and entry value-setting.
The `formatHeaders` static method is also altered to do a similar
deduplication of casing variants, but now additionally mentions
"with native header names [native name set]" if the native name set
contains casing variants.
Closes gh-33823
9b3cb15389