Prior to this commit, the entry set of read-only HttpHeaders lost the
original headers' ordering.
The changes in commit ce7278aaf4 introduced a regression in the read-only
HttpHeaders support. Specifically, the implementation of entrySet() in
the internal ReadOnlyHttpHeaders class converted the original entry set
to an immutable, non-ordered set of immutable entries.
This commit fixes this issue by converting the original entry set to an
immutable, ordered set of immutable entries.
Closes gh-23551
9729b460f1