Add tests for nAry, eq, where and propOrDefault

- Add new tests for nAry that didn't have tests before.
- Add test for currying eq.
- Add tests for eq, where and propOrDefault with 0 args.
- Fix eq to throw with 0 args.
We could use `curry2` here:

```javascript
R.eq = curry2(function(a, b) { return a === b; });
```