bug: Panic (index out of range) when humanizing whitespace

Each of these causes  a panic:

```go
fmt.Println(flect.Humanize(" "))
fmt.Println(flect.Humanize("\n"))
fmt.Println(flect.Humanize("\r"))
fmt.Println(flect.Humanize("\t"))
fmt.Println(flect.Humanize(" \n\r\t"))
```

