Fix error in isRFC3339_ISO6801

[ricochet1k]

Fixes TypeError: str.substring is not a function when large integers are returned from queries.

[retzkek]

Thanks for the patch, how big of an integer are we talking about? I wasn't able to reproduce it, even with MAX_SAFE_INTEGER. Could you add a test case (in util.test.ts) to reproduce it?

That said, we probably should have isRFC3339_ISO6801() only take string arguments since anything else doesn't make sense. On the caller side we could just wrap the argument with String() to be lazy, or do some more involved type-checking.

[carvid]

We hit that error today. It fails even with small integers (e.g. 1288310).

[retzkek]

Replaced by #33