Check that object is instance of Iterable in is_iterable

[aivrit]

Added an assertion to is_iterable that verifies the object is an instance of Iterable in addition to the built in python check.
If the assertion will fail the function will return False as expected.
This will fix issue #45
Thanks!

[insightindustry]

Thanks! This is a helpful suggestion, and I'd like to take a closer look at the PR. I'm merging the PR into a new branch for patch_is_iterable where I'd like to implement a variety of unit tests to ensure that a) we have good test coverage for this edge case, and b) it is functional across all the versions of Python supported by the Validator Collection.