Day-of-week parse support for MON-SUN (or `1-7`)

[stayclassychicago]

cronexpr.Parse() correctly parses dow (day-of-week) when written as 0-6 or SUN-SAT – but does NOT correctly parse when written as MON-SUN or 1-7

We can see that 0 and 7 both resolve to Sunday, so it should be possible to write expressions as either 0-6 OR 1-7 and have the same result. However- the 1-7 day-of-week does not parse.

Here is an example / reproduction scenario

package main

import (
	"fmt"
	"time"

	"github.com/gorhill/cronexpr"
)

func main() {
	t := time.Date(2017, time.July, 18, 13, 47, 0, 0, time.UTC)

	for _, dow := range []string{"MON-SUN", "SUN-SAT", "0-6", "1-7"} {
		times := cronexpr.MustParse(fmt.Sprintf("* *  * * %s", dow)).NextN(t, 8)

		if len(times) == 0 {
			fmt.Printf("Could NOT parse [%s]\n", dow)
		} else {
			for i := range times {
				fmt.Println(times[i].Format(time.RFC1123))
			}
		}

	}

}