Why jas donot have a default value in func RequireXXX

Some times default value is needed. But I have to use such code with jas.

```
str, err := ctx.FindString(paths...)
if err != nil {
    str = defaultStr
}
```

It is a little strange. Why cannot do it like that.

```
str := ctx.RequireDefaultString(defaultStr, paths...)
```
