Fix http auth header parsing (#34936)

Using `strings.EqualFold` is wrong in many cases.

modules/util/string.go

@@ -110,3 +110,24 @@ func SplitTrimSpace(input, sep string) []string {
 	}
 	return stringList
 }
+
+func asciiLower(b byte) byte {
+	if 'A' <= b && b <= 'Z' {
+		return b + ('a' - 'A')
+	}
+	return b
+}
+
+// AsciiEqualFold is from Golang https://cs.opensource.google/go/go/+/refs/tags/go1.24.4:src/net/http/internal/ascii/print.go
+// ASCII only. In most cases for protocols, we should only use this but not [strings.EqualFold]
+func AsciiEqualFold(s, t string) bool { //nolint:revive // PascalCase
+	if len(s) != len(t) {
+		return false
+	}
+	for i := 0; i < len(s); i++ {
+		if asciiLower(s[i]) != asciiLower(t[i]) {
+			return false
+		}
+	}
+	return true
+}