Humanize.truncateWords returns null when input has less number of words

```
Humanize.truncateWords("hello world", 2)
null
Humanize.truncateWords("hello world", 1)
"hello ..."
```

The primary reason one wants to use methods such as `truncateWords` is if the size increases beyond certain point, truncate it (and leave it untouched if input is smaller in length). Currently the method is returning `null` if input has less no. of words than requested. 
