expect(...).to.include should support ES2015 Sets and Maps

[mdziekon]

It seems like Chai supports key assertions on ES2015 sets & maps (introduced in this PR: #633), but it does not support simple "inclusion" assertions.

Consider the following testcase:

it("should detect item existence in Set", function () {
    let container = new Set();

    container.add(123);

    expect(container).to.include(123);
});

This test should obviously pass, as the container has 123 value added to it. Instead, it fails with the following error:

AssertionError: object tested must be an array, an object, or a string, but set given

Same thing happens if we change Set to Map (container.set("somekey", 123)).

---

As far as I know, this problem occurs both in 3.5.0 version, as well as the latest 4.0.0-canary.2 pre-release version.

[meeber]

@mdziekon Agreed. This one seems fairly straightforward.
@lucasfcosta @vieiralucas @shvaikalesh @keithamus Any objection to requesting a PR?

[lucasfcosta]

I totally agree. Seems simple, straightforward and also very useful.

[vieiralucas]

LGTM