fix expect throw syntax in tests.  change arguments to objects.

I was looking around for Bug Hunt candidates and noticed that a couple of the tests in ```string.js``` had syntax errors.

```expect(fn).to.throw;``` will not fail if ```fn()``` doesn't throw, it has to be called like ```expect(fn).to.throw();```

I also changed the arguments to Joi.string().valid() from a number to an object, because as far as I can tell a number shouldn't cause valid() to throw.