Allow objects with no prototype to tested against object literals.

Allow objects with no prototype to tested against object literals.

I often use objects created with `Object.create(null)` as dictionaries, since then they don't have pre-filled dictionary entries for e.g. `constructor`, `hasOwnProperty`, `isPrototypeOf`, etc. However, in my tests, it is quite useful to test the dictionaries I create against object literals (which have `Object.prototype` as their prototype).

This patch makes an exemption to the rule that two objects must have the same constructor, for the case where one has `null` prototype and the other `Object.prototype` as its prototype.