fix weekday durationToNextRun error

[xwjdsh]

What does this do?

fix #282

For the following test code, the duration of the durationToNextRun is 0s, then if I change it to s.Every(1).Hour()... or s.Every(1).Day()..., the duration is 24h0m0s / 1h0m0s, I think the logic of weekday duration calculation should same as day/hour and other units, add a cycle if nextRunTime equals lastRunTime.

package main

import (
	"fmt"
	"time"

	"github.com/go-co-op/gocron"
)

func main() {
	s := gocron.NewScheduler(time.UTC)
	s.StartAsync()

	t := time.Date(2022, 1, 24, 0, 0, 0, 0, time.UTC)

	s.Every(1). Monday().StartAt(t).Do(func() {})
	fmt.Println(s.NextRun())
	fmt.Println(s.NextRun())

	ch := make(chan bool)
	<-ch
}