Warn against a non-object expected value given to propEqual

[Krinkle]

propEqual (docs) is for the strict-equality comparing of own properties of two objects. When the actual value is a non-object, such as a string, it can act in somewhat unexpected ways.

For example:

const actual = 'fuh';
assert.propEqual(actual, 'foo');

severity: failed
actual: {
  "0": "f",
  "1": "u",
  "2": "h"
}
expected: {
  "0": "f",
  "1": "o",
  "2": "o"
}

This is because the values compared are the boxed-object equivalent of the strings. Similar to what Object('foo'), or new String('foo'), or 'foo'.split('') would produce.

---

Ref qunitjs/qunit#411 (comment).

[platinumazure]

Yes, let's create a new rule for this. Open to name suggestions.

I also think this probably should be added to recommended in a major release. I think it's probably too late to do this for the current upcoming major release but we'll see.

[platinumazure]

Working on this.

[platinumazure]

I've created #148 to implement this.