auth/rules: compare strings with strings.EqualFold

[Juneezee]

Comparing two strings to the same case with strings.ToLower is more computational expensive than strings.EqualFold.

Sample benchmark:

func BenchmarkToLower(b *testing.B) {
	for i := 0; i < b.N; i++ {
		if strings.ToLower("FOOBAR") != strings.ToLower("foobar") {
			b.Fail()
		}
	}
}

func BenchmarkEqualFold(b *testing.B) {
	for i := 0; i < b.N; i++ {
		if !strings.EqualFold("FOOBAR", "foobar") {
			b.Fail()
		}
	}
}

Result:

goos: linux
goarch: amd64
pkg: micro.dev/v4/util/auth/rules
cpu: AMD Ryzen 7 PRO 4750U with Radeon Graphics
BenchmarkToLower-16      	22834522	        99.96 ns/op	       8 B/op	       1 allocs/op
BenchmarkEqualFold-16    	151317182	         8.163 ns/op	       0 B/op	       0 allocs/op
PASS
ok  	micro.dev/v4/util/auth/rules	4.104s