Do not camelcase nested string values

Currently camelize has an issue where

``` js
camelize('foo-bar');
camelize({
  'foo-bar': 'baz-foo'
})
```

will return

```
'fooBar'
{
  fooBar: 'bazFoo' // expected fooBar: 'baz-foo'
}
```

This PR changes this by only camelcasing strings if they are the first value passed to camelize