titleize unexpected behavior with non-whitespace separators (underscores, dashes)

I was expecting titleize to work like Rails titleize, for example:

```
1.9.3p194 :003 > "foo-bar".titleize
=> "Foo Bar"
1.9.3p194 :004 > "foo_bar".titleize
=> "Foo Bar"
```

But in this library, the separator character is retained:

```
_.titleize('foo_bar')
"Foo_bar"
_.titleize('foo-bar')
"Foo-bar"
```