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

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

* gh-47005: fix do_open() to let regular headers override unredirected headers

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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Co-authored-by: Senthil Kumaran <senthil@python.org>