Inconsistent return value for shortFmt

### The current behavior

When given an Number less than 1000, shortFmt returns a Number. Otherwise, it returns a String.
### Why is this a problem?

Consider the following case:

```
{{ (n | shortFmt:1) || 'N/A'}}
```

When `n = 0`, this will print "N/A" (Why? Because 0 is a false-y value in JS. Therefore, it evaluates the right-hand side of the "||"). I believe that most developers would expect this to print "0" instead. (Similar to the behavior of angular's `number` filter)
### Proposed fix

When `n < 1000`, return `n + ''`. This will coerce the Number into a String.

If this seems like a reasonable behavior, then I'll go ahead and create a PR for this later today.