`MultiSelect` is too finicky about type of `default`

For example, this doesn't work:
```
MultiSelect([1=>"a", 2=>"b"]; default=Any[1])
```
![image](https://user-images.githubusercontent.com/4170948/177173753-cb898db8-f53f-41a9-8fbf-8234c5e576d7.png)

This can become annoying when the default is generated from some loosely-typed data.