expect(object).toBeObjectOf...();

[JamieMason]

Needs some more thought and discussion but, as it is quite common to create indexes of values by a given key, these could be a nice way of describing those indexes in a similar way to how we do with Arrays now.

expect(object).toBeObjectOf(any.iso8601());
expect(object).toBeObjectOfBooleans();
expect(object).toBeObjectOfNumbers();
expect(object).toBeObjectOfObjects();
expect(object).toBeObjectOfSize(number);
expect(object).toBeObjectOfStrings();