dateFormat doesn't work

```
<vue-monthly-picker
        v-model="search.date_from"
         :dateFormat="'YYYY-MM'"
         :clearOption=false
         :inputClass="'form-control form-control-sm rounded-right-0 cursor-pointer'"
 />

```

i got result like this　↓

```
...
date_from:{
 ....
 _d:  "YYYY/MM"
 _i:  "2020/5"
 ....
}
...
```

prop is passed correctly.