Fix substring matches in WWW-Authenticate parsing #3391

Job	Run time
checks / readme-snippets	11s
checks / test (3.14, lowest-direct, windows-latest)	1m 45s
checks / docs	39s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 5s
checks / pre-commit	52s
checks / test (3.13, lowest-direct, windows-latest)	2m 29s
checks / test (3.14, locked, windows-latest)	2m 34s
checks / test (3.10, locked, ubuntu-latest)	1m 20s
checks / test (3.14, locked, ubuntu-latest)	2m 14s
checks / test (3.10, lowest-direct, ubuntu-latest)	1m 13s
checks / test (3.13, locked, windows-latest)	1m 51s
checks / test (3.10, locked, windows-latest)	2m 7s
checks / test (3.12, locked, windows-latest)	2m 6s
checks / test (3.11, locked, ubuntu-latest)	1m 20s
checks / test (3.13, locked, ubuntu-latest)	1m 5s
checks / test (3.11, lowest-direct, ubuntu-latest)	1m 9s
checks / test (3.12, lowest-direct, ubuntu-latest)	1m 20s
checks / test (3.11, locked, windows-latest)	1m 56s
checks / test (3.10, lowest-direct, windows-latest)	1m 49s
checks / test (3.13, lowest-direct, ubuntu-latest)	1m 12s
checks / test (3.12, locked, ubuntu-latest)	1m 32s
checks / test (3.12, lowest-direct, windows-latest)	1m 52s
checks / test (3.11, lowest-direct, windows-latest)	2m 9s
all-green	5s
	35m 55s

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