[Fixes #1156] EmptyResult should not set status code (or do anything for that matter)

I tried to go that way by doing
```C#
new Mock<ActionContext>(MockBehavior.Strict, ...);

Assert.DoesNotThrow( () => emptyResult.ExecuteResult(context));
```

But ```ActionContext``` is not mock friendly as the properties need to be virtual, so I decided to go the other way around and check the status code, headers and body.

The default value for StatusCode is 200. If I were to chose a way of testing this, I would do it as I described above, and I think it's also valuable for our customers to be able to do Mock<ActionContext>() for the same reason (To easily test ExecuteResult on their action results). but that requires making the properties virtual.