Convert to string using rune() again

[QuLogic]

Not sure why this wasn't causing issues at the time of #101.

Also, I think the bounds check isn't necessary in this case because the Int64Val would have already done it, but I'm not certain.

[cosmos72]

The fragment you propose to modify is supposed to perform untyped integer -> untyped string conversion.

It must return a string, not a rune

[QuLogic]

Oops, I meant to wrap that in string again, sorry.