Misc/NEWS.d/next/Library/2020-12-04-03-51-12.bpo-5054.53StYZ.rst

CGIHTTPRequestHandler.run_cgi() HTTP_ACCEPT improperly parsed. Replace the
special purpose getallmatchingheaders with generic get_all method and add
relevant tests.

Original Patch by Martin Panter. Modified by Senthil Kumaran.