fix(switchCase): promise fall through

correctly return an Observable when the result is a Promise. The previous code ran 
```javascript
var result = sources[Promise];
isPromise(undefined) && (result = observableFromPromise(undefined));

return undefined || defaultSourceOrScheduler;
```
which then ran the default case
I don't get what sources[key] buys me here.  result is already transformed into a Promise by this: 
```js
isPromise(result) && (result = observableFromPromise(result));
```