fixing bug in testing for IterableDataset

[MikeScarp]

Before submitting

• Was this discussed/approved via a Github issue? (no need for typos, doc improvements) (sort of, see below)
• Did you read the contributor guideline?
• Did you make sure to update the docs? (not applicable here)
• Did you write any new necessary tests? (not applicable)

What does this PR do?

Not a top level issue, but fixes comments raised in #231 and #405 and came up in #323 .

PR review

Anyone in the community is free to review the PR once the tests have passed.
If we didn't discuss your PR in Github issues there's a high chance it will not be merged.

Did you have fun?

Make sure you had fun coding 🙃

[williamFalcon]

@MikeScarp thanks for the quick fix!