Fix #67792: HTTP Authorization schemes are treated as case-sensitive

We use `zend_binary_strncasecmp()` to avoid any locale issues, and
refactor. We also add a test case for Digest auth

Fix #67792: HTTP Authorization schemes are treated as case-sensitive

We use `zend_binary_strncasecmp()` to avoid any locale issues, and
refactor. We also add a test case for Digest authentication.

Closes GH-6900.

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