Logger is slow in OFF level

[vicanso]

I set the log's level to OFF and I thought that log function should be fast. However it seemed to be slower than of noop function. I have tested like this:

func BenchmarkEchoLog(b *testing.B) {
	e := echo.New()
	e.Logger.SetLevel(log.Lvl(log.OFF))
	c := e.NewContext(nil, nil)
	for i := 0; i < b.N; i++ {
		c.Logger().Debug("a")
	}
}

func BenchmarkNoop(b *testing.B) {
	noop := func(args ...interface{}) {}
	for i := 0; i < b.N; i++ {
		noop("a")
	}
}

BenchmarkEchoLog-4                  	 1000000	      1778 ns/op
BenchmarkNoop-4                     	30000000	        40.8 ns/op