Fix undefined behavior in cookie SameSite case-insensitive comparison.

Cookie::stricmp used tolower(str1[idx]) directly on char.
When char is signed and contains non-ASCII byte values, calling tolower this way is undefined behavior.

Cast each character to unsigned char before passing it to std::tolower, then cast back to char for comparison.

This makes SameSite string comparison standards-compliant and safe across platforms/char signedness, with no behavior change for normal ASCII inputs.

• Built unittest target.
• Ran: ./build/lib/tests/unittest -r CookieTest -s
• Result: All tests passed (12 assertions, 1 test case).
• Minimal, targeted change in cookie comparison logic only.
• No API changes.