Prior to this commit, getHeaderNames() returned duplicates for the
Content-Type and Content-Length headers if they were set in the wrapped
response as well as in ContentCachingResponseWrapper.
This commit fixes that by returning a unique set from getHeaderNames().
In addition, this commit introduces a new test in
ContentCachingResponseWrapperTests to verify the expected behavior for
Content-Type and Content-Length headers that are set in the wrapped
response as well as in ContentCachingResponseWrapper.
See gh-32039
See gh-32317
be45481d70