[3.15] gh-47005: fix do_open() to let regular headers override unredirected … (GH...

[3.15] gh-47005: fix do_open() to let regular headers override unredirected … (GH-146506) (#153377)

gh-47005: fix do_open() to let regular headers override unredirected … (GH-146506)

AbstractHTTPHandler.do_open() was building the request header dict by
starting with unredirected_hdrs and only inserting regular headers that
were not already present, giving unredirected headers priority.  This
contradicts get_header() and header_items(), both of which give regular
headers the higher priority.

Fix by unconditionally updating with req.headers so that a header set
via add_header() always overrides one set via add_unredirected_header().

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(cherry picked from commit 6d386684ad69f42cb57c72fe0e0ffcec82ec7e12)

Co-authored-by: CHINMAY <89741289+Das-Chinmay@users.noreply.github.com>
Co-authored-by: Senthil Kumaran <senthil@python.org>